There are numerous reports of this behavior on outdated forum versions (this was fixed in UBB.threads v7.5.8 and improved in v7.7.5; this happens when special/uft8 characters are inserted on PHP 5.4 or php.ini server configuration that HIDES the entire text block; they are now run through htmlspecialchars); the fix would be to upgrade your forums from your build, which was released in 2010, and to run the content rebuilders once you're running something that isn't severely outdated.
Some of the aforementioned reports:https://www.ubbcentral.com/forums/ubbthreads.php/topics/253582/empty-postshttps://www.ubbcentral.com/forums/ubbthreads.php/topics/256299/7-5-7-posts-disappering
Also, max_input_vars would only affect editing permissions on sites with a lot of forums, and would have no affect on the reported behavior.
Again, your UBB.threads version was released 10 years ago, before UTF8 became a standard; you should really keep up on updates to both PHP and UBB.threads as many bugs have been fixed in the last decade. There have also been significant security related changes to the UBB.threads package as well as within PHP in the last decade.