Mail app on OS X 10.10 goes blank with Office 365 Account after displaying for a minute
This seems to be a known bug which has been around since (if Google is anything to go by) at least OS X 10.8 (!) and is still not fixed. That's modern Apple for you.

None of these worked for me (e.g. the "Rebuild" option is greyed out, for starters!) but maybe they'll help you.

Mail.app inbox becomes blank after a few minutes of use