For the upstream kernel within 14.04.5 the problem is happening again.
In the kernel log I found issues with the i915 driver:

[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many full retries, give up
[drm:intel_wait_ddi_buf_idle [i915]] *ERROR* Timeout waiting for DDI BUF B idle bit

kernel: [599006.771813] WARNING: CPU: 0 PID: 914 at /build/linux-lts-xenial-Hu9lgy/linux-lts-xenial-4.4.0/drivers/gpu/drm/i915/intel_dp_mst.c:493 intel_dp_destroy_mst_connector+0x
cd/0xd0 [i915]()
kernel: [599006.771816] Disabling mst crtc failed with -22
kernel: [599006.771817] Modules linked in: xfrm_user xfrm_algo xt_addrtype br_netfilter aufs xt_nat xt_mark nf_conntrack_netbios_ns nf_conntrack_broadcast ipt_MASQUERADE nf_nat_ma
squerade_ipv4 xt_tcpudp ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_na
t_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw iptab
le_filter ip_tables x_tables arc4 cmac hp_wmi sparse_keymap snd_hda_codec_realtek snd_hda_codec_hdmi snd_hda_codec_generic snd_usb_audio snd_usbmidi_lib snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd
_seq_midi intel_rapl snd_seq_midi_event snd_rawmidi x86_pkg_temp_thermal intel_powerclamp coretemp uvcvideo videobuf2_vmalloc kvm_intel videobuf2_memops iwlmvm snd_seq snd_pcm videobuf2_v4l2 mac80211 kvm vid
eobuf2_core irqbypass v4l2_common videodev iwlwifi joydev snd_seq_device serio_raw media hid_plantronics snd_timer input_leds cfg80211 rtsx_pci_ms btusb btrtl memstick btbcm snd btintel rfcomm bnep soundcore
 bluetooth shpchp lpc_ich mei_me mei 8250_fintek mac_hid tpm_infineon hp_accel lis3lv02d hp_wireless input_polldev parport_pc ppdev lp parport drbg ansi_cprng dm_crypt hid_generic usbhid hid crct10dif_pclmul
 crc32_pclmul rtsx_pci_sdmmc i915 aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd i2c_algo_bit drm_kms_helper psmouse syscopyarea sysfillrect sysimgblt e1000e fb_sys_fops ahci drm libahci 
ptp rtsx_pci pps_core wmi video fjes
kernel: [599006.771937] CPU: 0 PID: 914 Comm: kworker/0:2 Tainted: G        W       4.4.0-34-generic #53~14.04.1-Ubuntu
kernel: [599006.771939] Hardware name: Hewlett-Packard HP EliteBook Folio 9480m/22DA, BIOS M85 Ver. 01.01 07/03/2014
kernel: [599006.771951] Workqueue: events drm_dp_destroy_connector_work [drm_kms_helper]
kernel: [599006.771954]  0000000000000000 ffff88024495fce8 ffffffff813d438c ffff88024495fd30
kernel: [599006.771958]  ffffffffc030c7c0 ffff88024495fd20 ffffffff8107d776 ffff880241726000
kernel: [599006.771961]  ffff8802408c0800 ffff880241471a50 ffff880241471650 ffff880241725810
kernel: [599006.771965] Call Trace:
kernel: [599006.771972]  [<ffffffff813d438c>] dump_stack+0x63/0x87
kernel: [599006.771978]  [<ffffffff8107d776>] warn_slowpath_common+0x86/0xc0
kernel: [599006.771981]  [<ffffffff8107d7fc>] warn_slowpath_fmt+0x4c/0x50
kernel: [599006.771991]  [<ffffffffc017c354>] ? drm_atomic_helper_set_config+0x94/0xb0 [drm_kms_helper]
kernel: [599006.772017]  [<ffffffffc007d76d>] ? drm_atomic_state_free+0x4d/0x70 [drm]
kernel: [599006.772052]  [<ffffffffc02c1f7d>] intel_dp_destroy_mst_connector+0xcd/0xd0 [i915]
kernel: [599006.772062]  [<ffffffffc017769b>] drm_dp_destroy_connector_work+0xab/0x150 [drm_kms_helper]
kernel: [599006.772067]  [<ffffffff81095970>] process_one_work+0x150/0x3f0
kernel: [599006.772071]  [<ffffffff810960ea>] worker_thread+0x11a/0x470
kernel: [599006.772074]  [<ffffffff81095fd0>] ? rescuer_thread+0x310/0x310
kernel: [599006.772078]  [<ffffffff8109b849>] kthread+0xc9/0xe0
kernel: [599006.772081]  [<ffffffff8109b780>] ? kthread_park+0x60/0x60
kernel: [599006.772085]  [<ffffffff817f770f>] ret_from_fork+0x3f/0x70
kernel: [599006.772088]  [<ffffffff8109b780>] ? kthread_park+0x60/0x60
kernel: [599006.772091] ---[ end trace 3af0cc086a51172a ]---
kernel: [599007.257246] [drm:intel_mst_disable_dp [i915]] *ERROR* failed to update payload -22
kernel: [599007.491996] [drm:intel_mst_pre_enable_dp [i915]] *ERROR* can't find connector