Forbidden You don’t have permission to access /vb/forumdisplay.php on this server.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request i did tail -f /usr/local/apache/logs/error_log and find lot of lines like this:
[Fri May 9 17:13:33 2008] [crit] [client] (24)Too many open files: /.htaccess pcfg_openfile: unable to check htaccess file, ensure it is readable
It seems the limit for open files is reaching. Type ulimit -a in shell. It should be 1024 by default.
You can add this limit for apache by adding this line to the apache init script and restarting apache.
# ulimit -n 4096