GoogleCalendar: After one hour, LoadCalendars fails with error 401: Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential

Hi!

I am using the Google-Calendar objects to read Calendars and read/write events.

Connecting and retrieving and editing works fine. For exactly an hour. After that, LoadCalendars (which I poll every minute) fails with error 401: Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential

What am I doing wrong here?

The basic overall workflow is this:
RefreshToken once
LoadCalendars (now once, until recently, I isseud LoadCalendars on every synchronization run)
download changed events every 30 seconds (using nextSyncToken)

Thanks for reading and any input
Stefan