Stock email app "Load More" feature not working

Recently, probably since the 5.1.1 update, but I don't remember when this first stopped working, the "Load More" feature has stopped working on the stock email app. If an email was too large to load fully, then the Load More button appears, but when I click on it the progress bar at the top animates for less than a second and then just stops with no more of the email loaded. Swiping up from the bottom of the email has the same "tries for a fraction of a second and then stops" effect.

Note that loading new emails is fine, but any emails that come in that are too large, I don't get any more of the message after the first download.

I cleared the caches on all of the variants of the stock email app in the app manager (there were 2 marked simply "Email", one for storage, one for sync, and one for compose) and rebooted the phone, but no luck.

I suppose the next step is to reset the data on those apps and recreate the account, but with several hundred emails on the server (my main computer is set to only delete messages after 3 weeks on the server so I can keep a backlog on my portable devices), I hesitate to resync them all and go through the process to delete the tons of messages I don't want any more so I'm hoping that there is a simpler solution (security certificates?).