We'll investigate that, it's possibly related to internal refresh token that happens every hour.
When token is about to expire (or app was turned off / sitting in tray) it will get refreshed immediately after you open it up.
By the way, is your OS clock and timezone up to date?
Since you said the error was persistent and only disappeared after a few minutes, it struck me that this could be a timing issue. Your token may have already expired, but if your computer clock was off, S3Drive might not have realized it needed to refresh it (edited)