Internal Server Error 500, File does not exist: /usr/local/apache/htdocs/

I have a problem, every install php program on my server,  error message .. ,

Code:

<br><p>The server encountered an internal error or
misconfiguration and was unable to complete
your request.</p>
<p>Please contact the server administrator,
 webmaster @ forum.afiscybermedia.com and inform them of the time the error occurred,
and anything you might have done that may have
caused the error.</p>
<p>More information about this error may be available
in the server error log.</p>
<p>Additionally, a 404 Not Found
error was encountered while trying to use an ErrorDocument to handle the request.<br><br>and, i view on my log ... <br><br>[Wed Nov 02 01:58:05 2011] [error] [client 182.3.122.40] SoftException in Application.cpp:601: Directory "/home/afiszone/public_html/forum" is writeable by group<br>[Wed Nov 02 01:58:05 2011] [error] Premature end of script headers: index.php<br>[Wed Nov 02 01:58:05 2011] [error] File does not exist: /home/afiszone/public_html/forum/500.shtml<br>[Wed Nov 02 01:58:14 2011] [error] File does not exist: /usr/local/apache/htdocs/Program<br>[Wed Nov 02 01:58:14 2011] [error]&nbsp; SoftException in Application.cpp:601: Directory "/home/afiszone/public_html/forum" is writeable by group<br>[Wed Nov 02 01:58:14 2011] [error] Premature end of script headers: index.php<br>[Wed Nov 02 01:58:14 2011] [error] File does not exist: /home/afiszone/public_html/forum/500.shtml<br>[Wed Nov 02 01:58:27 2011] [error] File does not exist: /usr/local/apache/htdocs/Program<br>[Wed Nov 02 01:58:40 2011] [error] File does not exist: /usr/local/apache/htdocs/Program<br>[Wed Nov 02 01:58:53 2011] [error] File does not exist: /usr/local/apache/htdocs/Program<br></p>