I’m using 1and1 for hosting and have been having this problem since I upgraded to 2.9. I was able to solve it today on 1and1 by changing the .htaccess in the wordpress installation folder to use php5 by default. I added this code at the top of the .htaccess file:
AddType x-mapp-php5 .php
It worked immediately. Even with all my plugins activated. Give this a shot if you are hosting with 1and1.
