I have the same issue with the current Ubuntu 18.04 kernel 4.15.0-23-generic, and also with ubuntu mainline kernel 4.17.0-041700, using Intel Wireless 8260 (Firmware 34.0.1) in AP mode.

Not sure if this is relevant or not: if i reboot the system, the wifi card vanishes completely (even from lspci), i always have to shut down and press the power button instead.

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BUG: unable to handle kernel NULL pointer dereference at 0000000000000068
IP: iwl_trans_pcie_txq_enable+0x62/0x460 [iwlwifi]
PGD 0 P4D 0 
Oops: 0002 [#1] SMP PTI
Modules linked in: ccm bridge stp llc pppoe pppox nf_conntrack_ipv6 nf_defrag_ipv6 ip6t_rt ip6table_filter ip6_tables nf_log_ipv4 nf_log_common xt_LOG ipt_REJECT nf_reject_ipv4 xt_policy xt_multiport xt_conntrack iptable_filter bnep ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack libcrc32c xt_TCPMSS xt_tcpudp iptable_mangle nls_iso8859_1 snd_soc_skl snd_soc_skl_ipc snd_hda_ext_core intel_rapl snd_soc_sst_dsp intel_telemetry_pltdrv snd_soc_sst_ipc intel_punit_ipc snd_soc_acpi intel_telemetry_core intel_pmc_ipc snd_soc_core x86_pkg_temp_thermal arc4 intel_powerclamp coretemp kvm_intel snd_compress ac97_bus kvm snd_hda_codec_hdmi irqbypass snd_hda_codec_realtek crct10dif_pclmul snd_hda_codec_generic crc32_pclmul ghash_clmulni_intel
 snd_pcm_dmaengine pcbc snd_hda_intel snd_hda_codec snd_hda_core iwlmvm snd_hwdep mac80211 aesni_intel aes_x86_64 crypto_simd glue_helper cryptd intel_cstate intel_rapl_perf iwlwifi snd_pcm btusb btrtl btbcm btintel serio_raw bluetooth snd_seq_midi snd_seq_midi_event ecdh_generic cfg80211 snd_rawmidi lpc_ich snd_seq snd_seq_device snd_timer snd mei_me mei mac_hid soundcore shpchp sch_fq_codel parport_pc ppdev lp parport ip_tables x_tables autofs4 uas usb_storage i915 i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops psmouse sdhci_pci drm sdhci r8169 ahci mii libahci i2c_hid hid video pinctrl_broxton
CPU: 3 PID: 5529 Comm: kworker/3:0 Not tainted 4.15.0-23-generic #25-Ubuntu
Hardware name: NA ZBOX-CI327NANO-GS-01/ZBOX-CI327NANO-GS-01, BIOS 5.12 01/16/2018
Workqueue: events iwl_mvm_add_new_dqa_stream_wk [iwlmvm]
RIP: 0010:iwl_trans_pcie_txq_enable+0x62/0x460 [iwlwifi]
RSP: 0018:ffffa38b41577c10 EFLAGS: 00010246
RAX: 00000000000009c4 RBX: 000000000000001f RCX: 0000000000000000
RDX: 0000000000000000 RSI: 000000000000001f RDI: 0000000000002710
RBP: ffffa38b41577c60 R08: 0000000000002710 R09: 0000000000000001
R10: 0000000000000000 R11: ffff8d6db6399fd0 R12: 0000000000000000
R13: ffff8d6db5aa0018 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8d6dbfd80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000068 CR3: 0000000136e0a000 CR4: 00000000003406e0
Call Trace:
 iwl_mvm_enable_txq+0x21a/0x3b0 [iwlmvm]
 iwl_mvm_add_new_dqa_stream_wk+0x809/0x1690 [iwlmvm]
 ? iwl_mvm_add_new_dqa_stream_wk+0x809/0x1690 [iwlmvm]
 ? __switch_to+0xad/0x500
 ? put_prev_entity+0x25/0x100
 process_one_work+0x1de/0x410
 worker_thread+0x32/0x410
 kthread+0x121/0x140
 ? process_one_work+0x410/0x410
 ? kthread_create_worker_on_cpu+0x70/0x70
 ret_from_fork+0x35/0x40
Code: 4c 8b b4 c7 e8 7d 00 00 f0 48 0f ab 87 e8 8d 00 00 73 0d 80 3d 16 ad 02 00 00 0f 84 c1 03 00 00 44 89 c7 e8 61 aa ac f6 4d 85 e4 <49> 89 46 68 0f 84 cf 02 00 00 41 0f b6 04 24 89 45 b8 41 0f b6 
RIP: iwl_trans_pcie_txq_enable+0x62/0x460 [iwlwifi] RSP: ffffa38b41577c10
CR2: 0000000000000068
---[ end trace 5d2a3090feef8ec6 ]---