I'm having exactly the same problem described by rnorth6920.
My forum is hosted by Dreamhost.
Error log:
Invalid query name 1
[Sat Sep 30 02:46:25 2006] [error] [client 86.112.236.184] Premature end of script headers: /dh/cgi-system/php.cgi
I've checked mod security and it is definitely disabled.
Was wondering if you found a solution to this, or do I need to contact my Web host and ask them to fix something?