Error 500 - Internal server error (1and1 hosting)

Copy and paste the following line into your .htaccess file:

go to http://drupal.org/node/232773 for the complete drupal forum post.

ADDITIONAL IMPORTANT INFORMATION

Link to the FAQ's page in 1and1.com:
http://faq.1and1.com//scripting_languages_supported/php/9.html

What do I need to consider if I want to use PHP 5?

With 1&1 Linux hosting packages you have the choice between PHP 4 and
PHP 5. To distinguish a script as PHP 5, simply name the script with the file
extension .php5.

While the PHP developers have done their utmost to ensure compatibility, we cannot
guarantee that PHP 4 scripts will function 100% correctly under PHP 5.

Please note that in contrast to PHP 4, the default for the variable RegisterGlobals
is "off".

By default Apache uses PHP 4 for .php extension. If you don't want to rename all your
scripts to .php5 you can do the following:
Create a .htaccess file and place the following line AddType x-mapp-php5 .php in it.

This will tell Apache to use PHP 5 instead of PHP 4 for the extension .php in the
directory the .htaccess is placed and all sub-directories under it.

You can use AddHandler x-mapp-php5 .php as an alternative for or instead of
AddType x-mapp-php5 .php