I have several blogs hosted by 1and1.com. I have been adding some plugins and kept coming across 500 Internal Server Error. I went back and forth, activating plugins, deactivating plugins and deleting them off of the FTP server. I was also coming across the WordPress blank screen on my admin dashboard. It was getting very frustrating. Then I came across something about one of the plugins generating the 500 Internal Server Error when the hosting company was not using PHP5. So I did some research into 1and1.com with WordPress and found a solution.
Find your .HTACCESS file on your server. It is a hidden file so make sure your FTP client (I use filezilla) can view hidden files. Open up the file in your text editor and add the following line after the line # END WORDPRESS.
AddType x-mapp-php5 .php
That’s it. Since I added this line I have not had an error 500 and all of my screens in WordPress have not been blank. Note: I did have one time when a plugin rewrote the .HTACCESS file. So keep a copy of the file handy. If you come across the error 500, just copy your backup of this file onto your FTP server.