Dotcom,
Sorry for the delay in getting around to testing this. Getting the 4.0 release out set us back slightly, but we're catching up.
I just tested by clicking on the German flag, then hitting Login With Facebook, and going through the normal registration process. When done, my profile was imported and I was successfully redirected to:
[removed]
I never saw the 500 error. I also did not log into the admin area to make sure everything was correct with the account there, but from the front-end, my profile was imported and things looked correct.
Please look at my account (testfb) and see if things look right to you, and let us know if you run into any other trouble.
Thanks,
Alex