i found the problem. it seems to be the login lockdown plugin. i disabled it, but maybe a contact to the author would be better? i had the problem. to find a problem next time, use a solution that is very similar to the problem you have. like login screens, lockdown and others can make your login screen malfunction, and when creating a user in mu (multi-user), something that is related to the database would cause it to stop working. like the plugin statpress it is not compatible with wordpress mu. it took me at least 1 hour to find out the problem.
