I've been experiencing the same thing on my 6+ running 8.1.3 too. I noticed it first started occurring on 8.1.2, where the mail screen would load to a blank white page, and would freeze the entire device. I had to do a hard restart in order to get the device to work again. It's still occurring intermittently on 8.1.3. Deleting/re-adding the accounts doesn't fix it.