If you are getting Error 500 messages on 1&1 this may be your fix…
It happened to work for me. The culprit is that 1&1 defaults to PHP version 4, not version 5. Try these lines in your .htaccess file:
AddType x-mapp-php5 .php
AddHandler x-mapp-php5 .php
You can read more about it here…
via Error 500 – Internal server error in WordPress with 1&1 | GP2B | Get Paid To Blog.