After updating the forum to version 2.6, posting/editing was broken.
Apparently this was caused by the Mark plugin no longer being compatible. Currently it is disabled.
Since I haven't caused the entire site to be rebuilt, old posts should remain unchanged, but any new posts (or edited posts) using the Mark facilities that it provided will no longer work.
Specifically this covers marking like:
==Highlighted Text==
Which now produces:
Highlighted Text
You can highlight using:
<ins>Highlighted Text</ins>
Highlighted Text