A few users have gotten a 500 error after installing WordPress for Joomla! and WordPress MU for Joomla! extensions. If you don’t know, a 500 Error is a server related error. I’ve been trying to figure out why this happens, and I think I’ve narrowed the issue down to a problem with the 1&1 hosting company.
Thankfully, one of the users just found a fix to this. All you have to do is put this line of code on your .htaccess file:
AddType x-mapp-php5 .php
It is as simple as that.
There is more information in 1&1 site. Our extension works on php4, but for some odd reason have to run it as php5 on the 1&1.