You can't just copy and paste an error message rather than ask us to view some screen shot (the domain blocked by my employers)? What have you tried to resolve the issue? Did you search the web before posting here? Perhaps paying for support is an option if you don't want to have to search for the solutions to problems?

"Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request."

This suggests to me that the ErrorDocument issue is in "Addition" to your internal server error.

You have been posting here and on other forums for years, you know the routine, (hint: How do I post a question effectively?).

Martin