1) This error can be at some servers of 1&1 hosting, or may be at some other ones.
For fixing this issue, please add this line to their .htaccess file:

AddType x-mapp-php5 .php

2) Also please try to deactivate one by one all your active plugins at the WordPress > Plugins menu. Then retest it again.
If its not help, please deactivate your active theme and active the default WordPress theme. And then retest it again. May be there exist some conflict with some active plugin or actual theme and just need to find the reason of that issue.

3) If the previous solutions will not help, please recheck your error.log file from your server configuration (please contact your server administrator or support of the hosting company, if you will have troubles with searching of this file) to be sure in a reason of that issue.