Wordpress plays nicely with most web hosts, although 1&1 are a little different.

If you try to update a freshly installed copy of WordPress on 1&1 hosting without making this change, some functions (mainly the automatic update processes) will not complete successfully.

The key is to edit the .htaccess file in the WordPress installation directory:

• FTP to your WordPress installation directory
• Download the .htaccess file
• Edit the file, adding the following line:
• AddType x-mapp-php5 .php
• Upload the file to the server, overwriting the old .htaccess file

WordPress updates should now complete without error.

You may also want to have a look at this post which deals with an error message after a failed update.