The re-direct has been left empty. I have no idea what went wrong but it works now again.
Btw how does it check if the user has already registered? Only on IP? Because we have University computers and people registering from those computers will all have the same IP. Then again, I had this setting on for about 2 years and there have never been any problems until now (although I must say that my service has seen a stark rise in popularity in the last months with about 1000 new registration per month).
Maybe the increased registration were the problems?