Replacing index.php with a simple new index.php that displays phpinfo() works. So this dosn't seem to be a permissions issue. AllowOverride All is in httpd.conf

/forum/ is owned by wwwrun and has write permissions. /forum/users/ also has write permissions and is owned by wwwrun.

I'm spending too much time on this (2 days into this) now. I'm getting ready to find some other option.