Are you able to pull the POP logs from the O365 side by chance? I am looking at a similar issue which arose over the weekend and everything I am seeing thus far points towards an index issue on the O365 side. To confirm though, you can head in to Settings>Diagnostics and check the POP logs and Ticket Import logs for more information. All of the session logs I've seen so far are successful and show no changes from previous POP download traffic. I hope this helps :)
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com