If you are getting Internal Server Error while doing /webmail /whm /cpanel , please check the apache error log.
For us it was like the below error
[Tue Oct 08 08:14:44 2013][error][client x.x.x.x] Premature end of script headers: wredirect.cgi [Tue Oct 08 08:14:44 2013][error][client x.x.x.x] File does not exist: /home/user/public_html/500.shtml
A temporary fix is to disable Suexec from WHM from the drop down at Home »Service Configuration »Configure PHP and suEXEC and save. But in this case you are compromising the security.
This is actually a known bug and if you are using cagefs then please do the below steps to fix this permanently
cagefsctl --remount-all service proxyexecd restart