AltGr not working most of the time

[Nervengift]

Setup is a Linux (nixos 20.03) VM on a Windows 10 laptop, usually with 2 screens attached. The VM gets 3 virtual screens. Both host and guest use a german (qwertz) keyboard layout.

Most of the time, the AltGr key and with it many symbols are not working on the guest. According to xev pressing the key does not trigger anything at all.

Often, but not always, changing from one virtual display to another will make the AltGr key work for a few times before falling back to not doing anything at all.

Sometimes, when changing to another screen and beginning to type, the first letter will be greek and I don't even know how to type those deliberately (it's not AltGr+something).

There have been days where it worked the whole time but those are rare.

Turning automatic keyboard capturing off as some related incidents suggested as solution does not help.

Suspending (save execution state) and restarting does also not help reliably.

To me it seems as if somehow the modifier gets stuck or a wrong keycode is sent or it gets out of sync between host and guest or between the virtual screens? I can't really find any 100% reliable way to trigger or fix this behaviour.

I'll gladly provide any additional info or perform tests that could help this issue.

[Nervengift]

virtualbox logfile

[Nervengift]

Update: I found out that dead_greek is mapped to Control_R on my system, which is also the host key.

Pressing Ctrl_R alone does not appear in xev but my usual Ctrl_R, then Super+Tab seems to register in the guest as Super down, dead_greek down, dead_greek up, Super up.

Not sure if this is related to this incident.