Very, very rarely I’ve seen “Authentication failed. Unexpected error. Contact support” on a regular poll and it recovers on the next poll. Maybe 2-3 times total. If you set the binding to DEBUG or (better yet TRACE) logging level and try again, there might be more context to see if there is anything in the binding or action bundle to address this. Hopefully this can be reproduced.
The message “Method failed: HTTP/1.1 500 Internal Server Error” occurs when tokens expire and need to be refreshed, so that’s usually normal.
You could also check to see if the ecobeeSetHold action returns true (success) or false (failed).