500 Errors on customers website..

[Scriptman]

Hello, i have read many 500 error topic's but nothing helped.
My problem is, i add myself as a customer with domain: aaa.nl (for example).
I have changed the DNS settings of that domain to my dedi server and uploaded my website to htdocs.
But all i get is an 500 error:
Error 500!
/
Internal Server Error.

I have the following spec's:
CPU: 2.2 Ghz
RAM: 2048 Mb
diskspace: 500 Gb (only 1.2Gb used)
Debian 4 (Etch)
I use ISPCP 1.0.0 Stable.
fcgid (I tried fastcgi, but then apache won't start)..

On my VPS server ISPCP 1.0.0 Stable works fine..

Is someone here that can help me with this little problem ?

Thanks.