Strange login problem – blank screen

When I go to login to my site by going to
http://www.mydomain.com/wp-login.php I see the usual login screen, put in the correct username and password but instead of seeing the dashboard at http://mydomain.com/wp-admin/ instead all I see is a blank page and the url in my browser remains as http://www.mydomain.com/wp-login.php

If I type wp-admin onto the end of the url after trying to login it takes me to my dashboard.

What seems to be happening is after I enter my username and password the script isnt sending me to http://mydomain.com/wp-admin/

Any idea what could be going on ?

This is affecting all my sites on one particular server.

Thanks for any help

Sean