Got the forum up...

Been adding/removing categories and having people join.

I got this error while I was adding and removing categories in the admin panel. When I go view the forum the error is there.

Fatal error: Call to undefined function mb_detect_encoding() in /codoforum/sys/Ext/simplehtmldom/simple_html_dom.php on line 1234

Thoughts on what it could be?

it's exactly what the error says it is, you would really need to show us whats on that line for us to help you. just by going what it says there though, you're either missing that function, or there is a error in the code, could be as simple as an open tag, or it could just plain be coded wrong. can't tell without seeing whats on that line though.