This is a common problem with those who are hosting with 1&1. Try adding this to the top of your .htaccess file and try again:
AddType x-mapp-php5 .php
500 Internal Server Error means that something in your site settings is causing the server to not want to serve your site
It could be in the htaqccess file, or it could be something as simple as a white space after a ?> closing tag in a php file, or it could be something far more deep-rooted.
Start by looking at the last change you made, or the htaccess file is a usual culprit.
Moderator
James Huff
(@macmanx)
Volunteer Moderator
Internal server errors are usually described in more detail in the server error log. If you have access to your server error log, generate the error again, note the date and time, then immediately check your server error log for anything useful. If you don’t have access to your server error log, ask your hosting provider to look for you.