Registering accounts works again
So it was not possible to create new accounts for some days now. I think I've fixed the problem now and everything should work as usual again.
Sorry for the problems.
UPDATE: My solution to the problem did not work. I temporarily turned off the captcha check when creating a new account. But this lead to massive spam. So've turned it back on again. I will try to find a working solution tomorrow.
UPDATE 2: I hope everything is fine again. I've still to keep an eye on the spam situation and the automated bots that seem to roam the net, but registering works again.