If domain is showing "Internal Server Error" except suspention page after suspending the domain then check sexec_log file. You will receive the following logs :
[2011-04-18 10:08:44]: uid: (1355/username) gid: (1355/username) cmd: suspendedpage.cgi
[2011-04-18 10:08:44]: error: target uid/gid (1355/1355) mismatch with directory (0/10) or program (0/0) or trusted user (0/10)
Then you need to correct the ownership of file suspendedpage.cgi to root.wheel
#chown root.wheel suspendedpage.cgi
This will fix the error.