Update: resolved since 2025-03-04T20:15:00Z
Since around 2025-03-04T07:00:00Z there have been issues that prevent Cloud from working and syncing correctly for some people.
SeriesGuide might show error messages for ex. when marking episodes watched or when syncing that it could not talk to Cloud.
The underlying issue appears to be an automatic scaling issue at Google Cloud. I hope they resolve this error soon. I will have more time to look into this tomorrow.