I have been on 2.5 since that was released, was on beta 2 before that. The last time it worked, as far as I can see was 18 February. I was migrated long before that so it did work with 2.5. I have tried rebooting OH, that didn’t help. Are you also using the binding and if so, what are you seeing as a value for the acces_token, is that null?
Did you also try cleaning the cache? FYI, it’s been posted several times in the forum that since 2.5 restarting OH several times is needed. For me it usually takes 2 restarts before things start working correctly.
So, when scrolling through the documentation one last time, I suddenly saw:
If you later change your client_id and client_secret in the configuration, you will have to locate and delete the file gcal_oauth2_token, and stop and restart the service. This is because the Google Calendar Scheduler does not detect that the OAuth token is no longer valid. On openHAB 2.0.0 installed via apt-get, this file is located in the directory /var/lib/openhab2/gcal
Whenever the error occurred, I did not change my secret/id but 2 weeks ago I did to debug this issue. So I removed the file and now it works again!