Joren,
We're at the Joomla World Conference this weekend, which is definitely slowing our response time down a little. I apologize for the delays, and we do have a 30-day money back guarantee. If you'd like to take advantage of that, please use the "Refunds" link at the bottom of this page.
As to your issue, I'm not really understanding the current issue right now. I just registered on the site without a problem. I was also able to login using the standard username/password option. Can you let me know:
* Is the K2 plugin currently disabled?
* What happens when you disable SEF URLs?
* Are you using a separate SEF extension, or Joomla's built-in SEF?
* Can you try switching the template to one of the built-in Joomla ones to test and see if that works?
Mainly, the 403 error page is odd because it's not reporting any sort of usable error to diagnose things. Because of that, we need to narrow down the issue a lot more manually by testing different things to see when it starts working and narrowing things down. Since I can recreate the error anymore, that also makes it more difficult.
Thanks,
Alex