The hard way: Surrender your profile.
Copy your profile to a safe place, delete & re-create your account, log in once, copy back all data except for the Microsoft
folders in AppData\Local
and AppData\Roaming
and the Registry. Only copy back the remaining stuff on a one-by-one basis, carefully checking whether you broke something and rolling back in case you did.
This solved the problem for me for now, but for future reference, I'm still looking for an answer.