This happens to me running 64-bit Xubuntu 15.10 on a Dell Latitude E7440, when unlocking the screen after it's been sent to sleep. Light Locker Settings claims "Your screensaver settings are managed by Xfce Power Manager [Open]"; opening that utility, it's not clear what I need to switch off to hand control back to Light Locker without losing other power management options. This might all be beside the point.

In any case, I'm tailing /var/log/syslog all the time; when I return to my machine after the screen has been switched off for a time, after presenting my credentials to unlock the running session, I'm presented with an apport dialog stating that I'm attempting a task that requires root privileges (I'm paraphrasing, I'll update with the full details later.)

/var/log/syslog contains the following stack trace:

Dec 16 15:55:08 gd2pd12 org.gtk.vfs.Daemon[21443]: A connection to the bus can't be made
Dec 16 15:55:08 gd2pd12 kernel: [597970.459037] [drm:intel_pipe_config_compare [i915]] *ERROR* mismatch in ips_enabled (expected 1, found 0)
Dec 16 15:55:08 gd2pd12 kernel: [597970.459047] ------------[ cut here ]------------
Dec 16 15:55:08 gd2pd12 kernel: [597970.459091] WARNING: CPU: 3 PID: 5803 at /build/linux-26_gwp/linux-4.2.0/drivers/gpu/drm/i915/intel_display.c:12328 check_crtc_state+0x2c5/0x440 [i915]()
Dec 16 15:55:08 gd2pd12 kernel: [597970.459095] pipe state doesn't match!
Dec 16 15:55:08 gd2pd12 kernel: [597970.459098] Modules linked in: btrfs xor raid6_pq ufs qnx4 hfsplus hfs minix ntfs msdos jfs xfs libcrc32c xt_tcpudp xt_REDIRECT nf_nat_redirect xt_hl snd_usb_audio snd_usbmidi_lib binfmt_misc nls_utf8 isofs cdc_acm uas usb_storage nfnetlink_queue nfnetlink_log nfnetlink veth xt_addrtype br_netfilter bridge stp llc aufs rfcomm drbg ansi_cprng ctr ccm iptable_filter xt_conntrack ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack ip_tables x_tables pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) bnep rt2800usb rt2x00usb rt2800lib rt2x00lib crc_ccitt arc4 pn544_mei mei_phy pn544 hci nfc dell_wmi sparse_keymap dell_laptop dcdbas intel_rapl iosf_mbi x86_pkg_temp_thermal intel_powerclamp dell_smm_hwmon coretemp uvcvideo videobuf2_vmalloc kvm_intel videobuf2_memops videobuf2_core v4l2_common videodev media kvm crct10dif_pclmul btusb btrtl crc32_pclmul btbcm btintel bluetooth iwlmvm aesni_intel mac80211 aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd iwlwifi input_leds serio_raw snd_hda_codec_realtek snd_hda_codec_hdmi snd_soc_rt5640 snd_soc_rl6231 snd_hda_codec_generic cfg80211 snd_hda_intel snd_hda_codec snd_soc_core lpc_ich snd_compress snd_hda_core ac97_bus snd_pcm_dmaengine shpchp snd_hwdep mei_me snd_seq_midi mei snd_pcm snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer snd snd_soc_sst_acpi soundcore dw_dmac 8250_fintek dw_dmac_core dell_smo8800 i2c_designware_platform 8250_dw i2c_designware_core spi_pxa2xx_platform dell_rbtn joydev mac_hid parport_pc ppdev lp parport autofs4 hid_generic usbhid i915 ahci libahci psmouse sdhci_pci i2c_algo_bit drm_kms_helper e1000e drm ptp pps_core wmi video sdhci_acpi sdhci i2c_hid hid
Dec 16 15:55:08 gd2pd12 kernel: [597970.459338] CPU: 3 PID: 5803 Comm: Xorg Tainted: G        W  OE   4.2.0-19-generic #23-Ubuntu
Dec 16 15:55:08 gd2pd12 kernel: [597970.459343] Hardware name: Dell Inc. Latitude E7440/06MFX3, BIOS A08 02/18/2014
Dec 16 15:55:08 gd2pd12 kernel: [597970.459348]  0000000000000000 000000006dd48220 ffff8804076077c8 ffffffff817e93f9
Dec 16 15:55:08 gd2pd12 kernel: [597970.459355]  0000000000000000 ffff880407607820 ffff880407607808 ffffffff8107b3d6
Dec 16 15:55:08 gd2pd12 kernel: [597970.459361]  ffff880407607848 ffff8804076078b0 ffff880406df4000 0000000000000001
Dec 16 15:55:08 gd2pd12 kernel: [597970.459368] Call Trace:
Dec 16 15:55:08 gd2pd12 kernel: [597970.459381]  [<ffffffff817e93f9>] dump_stack+0x45/0x57
Dec 16 15:55:08 gd2pd12 kernel: [597970.459390]  [<ffffffff8107b3d6>] warn_slowpath_common+0x86/0xc0
Dec 16 15:55:08 gd2pd12 kernel: [597970.459397]  [<ffffffff8107b465>] warn_slowpath_fmt+0x55/0x70
Dec 16 15:55:08 gd2pd12 kernel: [597970.459440]  [<ffffffffc022a941>] ? intel_pipe_config_compare+0xb31/0xc60 [i915]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459475]  [<ffffffffc022ad35>] check_crtc_state+0x2c5/0x440 [i915]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459518]  [<ffffffffc023eb5d>] intel_modeset_check_state+0x21d/0x6d0 [i915]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459545]  [<ffffffffc023fd17>] intel_crtc_set_config+0x4c7/0x580 [i915]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459570]  [<ffffffffc0092ef5>] ? drm_mode_create+0x25/0x60 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459589]  [<ffffffffc008c4a6>] drm_mode_set_config_internal+0x66/0x100 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459610]  [<ffffffffc0090b39>] drm_mode_setcrtc+0x3e9/0x500 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459623]  [<ffffffffc0081495>] drm_ioctl+0x125/0x610 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459641]  [<ffffffffc0090750>] ? drm_mode_setplane+0x1b0/0x1b0 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.459647]  [<ffffffff810ae619>] ? update_curr+0x79/0x150
Dec 16 15:55:08 gd2pd12 kernel: [597970.459653]  [<ffffffff81210a25>] do_vfs_ioctl+0x295/0x480
Dec 16 15:55:08 gd2pd12 kernel: [597970.459658]  [<ffffffff81210c89>] SyS_ioctl+0x79/0x90
Dec 16 15:55:08 gd2pd12 kernel: [597970.459665]  [<ffffffff817f01f2>] entry_SYSCALL_64_fastpath+0x16/0x75
Dec 16 15:55:08 gd2pd12 kernel: [597970.459669] ---[ end trace 9d9e9b79204a234d ]---
Dec 16 15:55:08 gd2pd12 kernel: [597970.459821] [drm:intel_pipe_config_compare [i915]] *ERROR* mismatch in ips_enabled (expected 1, found 0)
Dec 16 15:55:08 gd2pd12 kernel: [597970.459825] ------------[ cut here ]------------
Dec 16 15:55:08 gd2pd12 kernel: [597970.459851] WARNING: CPU: 3 PID: 5803 at /build/linux-26_gwp/linux-4.2.0/drivers/gpu/drm/i915/intel_display.c:12328 check_crtc_state+0x2c5/0x440 [i915]()
Dec 16 15:55:08 gd2pd12 kernel: [597970.459853] pipe state doesn't match!
Dec 16 15:55:08 gd2pd12 kernel: [597970.459855] Modules linked in: btrfs xor raid6_pq ufs qnx4 hfsplus hfs minix ntfs msdos jfs xfs libcrc32c xt_tcpudp xt_REDIRECT nf_nat_redirect xt_hl snd_usb_audio snd_usbmidi_lib binfmt_misc nls_utf8 isofs cdc_acm uas usb_storage nfnetlink_queue nfnetlink_log nfnetlink veth xt_addrtype br_netfilter bridge stp llc aufs rfcomm drbg ansi_cprng ctr ccm iptable_filter xt_conntrack ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack ip_tables x_tables pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) bnep rt2800usb rt2x00usb rt2800lib rt2x00lib crc_ccitt arc4 pn544_mei mei_phy pn544 hci nfc dell_wmi sparse_keymap dell_laptop dcdbas intel_rapl iosf_mbi x86_pkg_temp_thermal intel_powerclamp dell_smm_hwmon coretemp uvcvideo videobuf2_vmalloc kvm_intel videobuf2_memops videobuf2_core v4l2_common videodev media kvm crct10dif_pclmul btusb btrtl crc32_pclmul btbcm btintel bluetooth iwlmvm aesni_intel mac80211 aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd iwlwifi input_leds serio_raw snd_hda_codec_realtek snd_hda_codec_hdmi snd_soc_rt5640 snd_soc_rl6231 snd_hda_codec_generic cfg80211 snd_hda_intel snd_hda_codec snd_soc_core lpc_ich snd_compress snd_hda_core ac97_bus snd_pcm_dmaengine shpchp snd_hwdep mei_me snd_seq_midi mei snd_pcm snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer snd snd_soc_sst_acpi soundcore dw_dmac 8250_fintek dw_dmac_core dell_smo8800 i2c_designware_platform 8250_dw i2c_designware_core spi_pxa2xx_platform dell_rbtn joydev mac_hid parport_pc ppdev lp parport autofs4 hid_generic usbhid i915 ahci libahci psmouse sdhci_pci i2c_algo_bit drm_kms_helper e1000e drm ptp pps_core wmi video sdhci_acpi sdhci i2c_hid hid
Dec 16 15:55:08 gd2pd12 kernel: [597970.460002] CPU: 3 PID: 5803 Comm: Xorg Tainted: G        W  OE   4.2.0-19-generic #23-Ubuntu
Dec 16 15:55:08 gd2pd12 kernel: [597970.460005] Hardware name: Dell Inc. Latitude E7440/06MFX3, BIOS A08 02/18/2014
Dec 16 15:55:08 gd2pd12 kernel: [597970.460007]  0000000000000000 000000006dd48220 ffff8804076077c8 ffffffff817e93f9
Dec 16 15:55:08 gd2pd12 kernel: [597970.460012]  0000000000000000 ffff880407607820 ffff880407607808 ffffffff8107b3d6
Dec 16 15:55:08 gd2pd12 kernel: [597970.460016]  ffff880407607848 ffff8804076078b0 ffff880406df4000 0000000000000001
Dec 16 15:55:08 gd2pd12 kernel: [597970.460021] Call Trace:
Dec 16 15:55:08 gd2pd12 kernel: [597970.460026]  [<ffffffff817e93f9>] dump_stack+0x45/0x57
Dec 16 15:55:08 gd2pd12 kernel: [597970.460032]  [<ffffffff8107b3d6>] warn_slowpath_common+0x86/0xc0
Dec 16 15:55:08 gd2pd12 kernel: [597970.460037]  [<ffffffff8107b465>] warn_slowpath_fmt+0x55/0x70
Dec 16 15:55:08 gd2pd12 kernel: [597970.460062]  [<ffffffffc022a941>] ? intel_pipe_config_compare+0xb31/0xc60 [i915]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460085]  [<ffffffffc022ad35>] check_crtc_state+0x2c5/0x440 [i915]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460115]  [<ffffffffc023eb5d>] intel_modeset_check_state+0x21d/0x6d0 [i915]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460141]  [<ffffffffc023fd17>] intel_crtc_set_config+0x4c7/0x580 [i915]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460164]  [<ffffffffc0092ef5>] ? drm_mode_create+0x25/0x60 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460183]  [<ffffffffc008c4a6>] drm_mode_set_config_internal+0x66/0x100 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460203]  [<ffffffffc0090b39>] drm_mode_setcrtc+0x3e9/0x500 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460216]  [<ffffffffc0081495>] drm_ioctl+0x125/0x610 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460233]  [<ffffffffc0090750>] ? drm_mode_setplane+0x1b0/0x1b0 [drm]
Dec 16 15:55:08 gd2pd12 kernel: [597970.460238]  [<ffffffff810ae619>] ? update_curr+0x79/0x150
Dec 16 15:55:08 gd2pd12 kernel: [597970.460244]  [<ffffffff81210a25>] do_vfs_ioctl+0x295/0x480
Dec 16 15:55:08 gd2pd12 kernel: [597970.460249]  [<ffffffff81210c89>] SyS_ioctl+0x79/0x90
Dec 16 15:55:08 gd2pd12 kernel: [597970.460255]  [<ffffffff817f01f2>] entry_SYSCALL_64_fastpath+0x16/0x75
Dec 16 15:55:08 gd2pd12 kernel: [597970.460258] ---[ end trace 9d9e9b79204a234e ]---


Here's the output from lspci:

 lspci
00:00.0 Host bridge: Intel Corporation Haswell-ULT DRAM Controller (rev 0b)
00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 0b)
00:03.0 Audio device: Intel Corporation Haswell-ULT HD Audio Controller (rev 0b)
00:14.0 USB controller: Intel Corporation 8 Series USB xHCI HC (rev 04)
00:16.0 Communication controller: Intel Corporation 8 Series HECI #0 (rev 04)
00:16.3 Serial controller: Intel Corporation 8 Series HECI KT (rev 04)
00:19.0 Ethernet controller: Intel Corporation Ethernet Connection I218-LM (rev 04)
00:1b.0 Audio device: Intel Corporation 8 Series HD Audio Controller (rev 04)
00:1c.0 PCI bridge: Intel Corporation 8 Series PCI Express Root Port 1 (rev e4)
00:1c.3 PCI bridge: Intel Corporation 8 Series PCI Express Root Port 4 (rev e4)
00:1c.4 PCI bridge: Intel Corporation 8 Series PCI Express Root Port 5 (rev e4)
00:1d.0 USB controller: Intel Corporation 8 Series USB EHCI #1 (rev 04)
00:1f.0 ISA bridge: Intel Corporation 8 Series LPC Controller (rev 04)
00:1f.2 RAID bus controller: Intel Corporation 82801 Mobile SATA Controller [RAID mode] (rev 04)
00:1f.3 SMBus: Intel Corporation 8 Series SMBus Controller (rev 04)
02:00.0 Network controller: Intel Corporation Wireless 7260 (rev 73)
03:00.0 SD Host controller: O2 Micro, Inc. SD/MMC Card Reader Controller (rev 01)

I'm running the following kernel:

$ uname -srvmpio
Linux 4.2.0-19-generic #23-Ubuntu SMP Wed Nov 11 11:39:30 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux