If you have made changes on the file, upload the original files back to ftp.
Especially the includes folder upload.
If you haven't changed, an incompatible product may be blocking. To find out it is incompatible product, apply in
this message
Check for setting IMG bbcode use on or off in each forum.
See each User Group IMG bbcode on or off setting.
If none, do as in screenshot
go to settings
your-site.ext/admincp/queries.php?do=modify