Some domains need a significant period of time to load and eventually fail with a '500 Internal Server error'. In the domain specific log
/var/www/vhosts/domain.tld/logs/error_log, there are the following message:
[Thu Oct 10 12:52:18 2013] [warn] [client 18.104.22.168] mod_fcgid: can't apply process slot for /var/www/cgi-bin/cgi_wrapper/cgi_wrapper
If there are
'can't apply process slot for' error messages in the error log, this means the
FcgidMaxProcesses limit has been met. This is the total number of processes that FCGID will start, for all users. That is what will prevent OOM (out of memory) issues. The more RAM the server has, the higher the value can be set. If this value is too low, there will be 'internal' errors (500), as FCGID will not be able to create new processes to serve requests. This value also depends on the size of PHP processes (which in turn depends on which extensions that are enabled for PHP). The larger the processes, the faster OOM will be hit.
To fix the issue please apply the article on the affected Apache service node.