Can't post when logged on

[Flying Dutchman]

Trying to post in any thread, both via quick reply and reply button, results in:

Fatal error: Call to a member function on a non-object in /home/mith/public_html/forum/includes/functions_post.php on line 366

when you hit 'submit'. The post is submitted, though............

I know I'm not the only one with this strange bug, but I'm the only one who posted the same post 7 times in a row because of it.......