pollMonitoredInboxes unable to import email with UID 40: the single id was not found in response

I've been getting a bunch of errors on pollMonitoredInboxes and I'm at a loss as to how to track what the problem is.

They all look like this: with different UIDs

[FATAL] pollMonitoredInboxes unable to import email with UID 40: the single id was not found in response

Any thoughts?

thanks,
Francesca