Unsuspicious Wrote: Disable all of your plugins and re-enable them one by one; keep refreshing the thread every time you re-enable one, until the 500 error occurs again.
Then you know what plugin is causing it, or whether a plugin is actually causing it.
It would be advised to make a backup of your forum, then instead of disabling them, the codes still in the templates so uninstall them one by one like above^ but once you get to the plugin, load up the backup and uninstall that one plugin... yes it takes a bit more time but it's more efficient