AltGr randomly stops working on windows 10

The issue happens when you open a RDP connection. Just bring the RDP window in foreground and press Alt+Enter.

The issue will disappear on all other windows too

Source: How to Fix Alt Gr Key Not Working.

In my case left Shift+Caps Lock+AltGr helped. I have no idea what it does, but it works.

Some utility programs can also "steal" the keyboard stroks.

In my case it appeared that the VirtualDesktopManager resulted in non-functional AltGr, so I could not write [ and ].