I was getting this issue too with 1and1 hosted sites:
In my case when i clicked the upgrade link from wp-admin i got a 500 error from 1and1.
Error 500 - Internal server error An internal server error has occured! Please try again later.
Things i tried:
- disabled all plugins (didnt work)
- I added the plugin for memory (didnt work)
This method did work for me:
Added "AddType x-mapp-php5 .php" to the .htaccess (in the main wp folder) file right before the </IfModule>, Saved and then clicked the upgrade link again.
Worked like a charm.
Just had to do a db update when the screen promoted me to. I think this is standard with the upgrade though.
Hope this helps people that are still having issues.