Why do I get - 500 Error - Internal Server Error when trying to install a package via RHN Satellite v5.4.1 web-ui

Issue

Why do we get the following error:

"500 Error - Internal Server Error

You may have reached this page in one of the following ways:

    You've found an error in the site. Please report this error to your local administrator with details of how you received this message."

when attempting to install a package on a server.

The 500 Error page doesn't come up for all servers, but it does happen for more than one. How do I fix it ?