@Dominik-Pellkofer, thank you for reporting this. You're now the 3rd person to report it. I'm working on it now.
Which Google action are you using?
TRIGGERcmd (ask TRIGGERcmd to run 😵 TRIGGER command (ask TRIGGER command to run 😵 TRIGGERcmd Smart Home (turn on 😵It seems Google changed something in their oauth2 authentication system that's causing problems with TRIGGERcmd. Last night I found an error like this in the logs on the Google side. https://github.com/actions-on-google/smart-home-nodejs/issues/388
I made a change to the TRIGGERcmd and TRIGGER command actions which might fix them. The change only takes effect next time people link their accounts. Unfortunately it doesn't happen very often, so only time will tell if it's fixed. In fact for some reason it's never happened to me yet.
I'll be working on this this weekend. Please keep reporting the problem when you see it. If you tell me the exact time it might help.
EDIT: I think I found the root cause and fixed it today, 9/12/2020 at about 12:30pm EST. You might have to re-link one more time, but after that it should stay linked.
Thank you for reporting the problem. After adding logging for this problem I see it was also affecting Alexa users.