Wifi (Intel Wireless AC 8260) only works when ethernet ist plugged in

A strange problem. Wifi only seems to work, when I boot Ubuntu with the ethernet cable plugged in. When the cable is not plugged in, Ubuntu behaves like there is no physical Wifi card/module.

This boot-with-ethernet-cable procedure doesn't work, when the cable isn't connected to a router/network on the other end.

After booting, the ethernet cable can be unplugged and Wifi continues to work just fine.

When I boot without ethernet cable dmesg then gives me:

[    9.108620] iwlwifi 0000:04:00.0: enabling device (0000 -> 0002)
[    9.156833] iwlwifi 0000:04:00.0: loaded firmware version 36.ca7b901d.0 8000C-36.ucode op_mode iwlmvm
[    9.422120] iwlwifi 0000:04:00.0: Detected Intel(R) Dual Band Wireless AC 8260, REV=0x204
[   11.522439] iwlwifi 0000:04:00.0: Couldn't prepare the card
[   11.522444] iwlwifi 0000:04:00.0: Error while preparing HW: -110
[   12.056146] iwlwifi 0000:04:00.0: Master Disable Timed Out, 100 usec

Testet on a fresh installed Ubuntu 21.10 with all packages updated.

I tried older ucode versions by renaming the newer ones but to no avail. Unloading and loading the the modules iwlmvm and iwlwifi with rmmod und modprobe after startup gives the same errors.

rfkill shows no hard or soft lock.

I additionally tested on Ubuntu 20.04 and Manjaro with the same outcome. Curiously, everything works out of the box on Windows and Arch (with the same driver version).

What could be the culprit here? What does Error Code -110 mean? How could I investigate further?

There is little info on your problem. This reports exactly the same, with no solution. This thread about a driver bug may be related, and it suggests some tweaking with power management might help. (This article in German might help, but I didn't translate it).

So I suggest you check the options of the driver with modinfo iwlwifi. Then tinker a bit with them. You can add line

options iwlwifi <option 1 name>=<option 1 value> <option 2 name>=<option 2 value>  ...

to file /etc/modprobe.d/iwlwifi.conf, to change driver options. You have to reboot for options to apply. I am not sure removing (sudo modprobe -r iwlwifi) and inserting (sudo modprobe iwlwifi 11n_disable=1 or whatever options) the driver module is enough.

For instance, I would try 11n_disable=1 or 8, bt_coex_active=0, power_save=0, power_level=3, disable_11ac=1, disable_11ax=1, remove_when_gone=0, one at a time.

Please post in the meantime the output (when WiFi works and when it doesn't) of:

$ lspci -knn | grep -E 'Net|Eth' -A4 
$ modinfo iwlwifi
$ inxi -F

Also, try an Ubuntu Live USB session.