This just started happening on my wifes 920 that runs the WP8.1 dev preview. It started on Sunday for her the 17th for no explainable reason. This was with her work email that sits on Exchange 2010 and had been working since she got the phone 18 months ago.
I removed and re-added the account which hasn't resolved it and I am unable to add her account to my 930 running the WP8.1 dev preview also. Other users at her work are seeing the same things on WP8.1 dev preview however those on 8.0 are not.
The only solution at the moment seems to be to wipe her phone back to 8.0 which I am very reluctant to do.
Something, somewhere has changed on Sunday/Monday to cause this problem with the WP8.1 dev preview.
Any suggestions or help?