I dont know your all accounts same or only for parked domains ?
If all accounts same on the server follow below:
please remove index.php from customer account and upload index.html so lets see. If you still receive 500 error or not. If you dont receive 500 error any more after the index.html upload this mean you have apache+php compile problem so build your php and apache again with whm.
Code:
http://Your_ServerIP:2086/scripts2/buildapache
After the complete apache build if you still receive 500 error, create a new account on whm and try upload any index.html file check error. If there is no error, take backup for customer then terminate customer account and open again.
If you still cant solve this issue, please provide your os and cpanel version on this topic.
If only parked domains:
If your cpanel version is 11 currently or edge turn back to stable version I suggest. Also check apache logs for problem parked domain.