Admin for users gives infinite loop

I´m sure I read about this somewhere, but can´t seem to find it..

I have a WPMU install on my localhost for a client of mine. Last week they gave me the login details for their VPS, so I moved the install to the VPS (did a fresh install and copied DB stuff).
Because of some extra options of MU2.7 (ie the upload-dir location) I upgraded the install from 2.6.5 to 2.7. So far so good (apart from the backend look.. the mu2.7 totally screwed up my custom admin look).

I made some minor tweaks to wp-admin (to not include the 2.7 css when it's not the site admin) and let it rest for a few days.
Last weekend I found out that the backend wasn't what it should be (apart from the fact that it was 2.7), the menu-item 'Blog account' (pay to blog) wasn't there and some default tabs were (which were supposed to be hidden by the Menus plugin).

Because of the troubles (backend, css) I decided to downgrade the install. Cleaned the root, dropped all tables in the database and re-moved the localhost install. From that point I was getting an infinite loop on wp-admin (the login screen worked fine) for all users. After re-installing (dropping everything again, doing a fresh install / moving the localhost install to the VPS) the problem just wouldn't go away. So, fresh install or not: problem of infinite loop on wp-admin stays.

I found out that I can only access the backend if the user has been added to the main-blog (that's when I get the default tabs and no Blog account tab in the admin menu). When I delete the user from the main-blog the infinite loops occurs. Trying to access blog.php (pay-to-blog) I get the message 'Nice try..'. Accessing files that I integrated into pay-to-blog gives me the message that it couldn't be loaded. And yes, those files do exist.

What I find weird about this situation is that everything works perfectly on my localhost (XAMPP btw).

Oh, right: the error-log doesn't give any information (nothing is added to the file when I get the loop). I already cleaned my cookies (that's when I found out that I could access with the site-admin account).

Also tried with plugins and mu-plugins folder empty, but to no avail.

Does anyone know what's happening here and how I can fix this?..