Have you made any recent character encoding changes such as from Windows-1252 to let's say UTF-8? If not then this could be modification related, I would log into the admincp then simply disable your mods one-at-a-time (each time you disable a mod then go back to refresh the forum page, see if the xml error is gone, when it's gone you will know which mod is the culprit).
Edit: Possibly related -
https://forum.vbulletin.com/forum/vb...error?t=106696