So I've looked into the issue.
Turns out, it has nothing to do with our boards, our servers, or anything on our end.
The issue is with imgur itself.
For whatever reason, they've changed their API coding to replace embedded galleries with block-quotes instead. Why they've done this? No clue, but they likely didn't realize there were third party sites using their API to display galleries, so they didn't know changing their scripts would effect anything.
As it stands, imgur simply does not support iframe embedding at this time, so we can't directly embed them into posts like we used to, regardless of what we do.