And you have no plugins active or plugin files uploaded?? If there was no field there, and no files trying to laod it, there wouldn't be anything to make this error happen...
its definately none of the current plugins i have, but im not sure if i still have a plugin file somewhere in my ftp. so im stuck ):
I know this is a old thread but I had the exact same problem took me forever to solve here is the solution go the the files for mybb on your server navigate to admin then modules then forum then open management.php it ctrl-F to bring up the find box then search for forum_icon and delete all the code for forum_icon also make sure you backup the file just in case before editing and this should solve your problem.