Hello to all.
A few months ago, the company where I have my sites performed an update to the servers, and some of my sites (mostly those with joomla 3) suddenly started showing the same error "500 - Internal Server Error".
After some research, I was able to solve the problem by creating a file with the name ".user.ini" (without quotes of course, but don't forget the dot before 'user'), and in the file simple put the line:
magic_quotes_gpc = Off
And the sites became active again.
Maybe it will work for you, hope I could help.