unfortunately, right after the activation of the new forums, some friendly users made me aware of a few issues. Upon closer inspection, I realized that the (new) database was partially corrupted. Do not worry, tho: The original forum content is still there and secured. Right now I am working hard to get the forums back up and running. In case it proves impossible to migrate to bbPress without corruptions, I will settle for something else. However this is hard to say right now. But until I can say something more specific, I will revert the site back to normal and keep the original forums closed.
I am sorry for the inconvenience and thank you for your patience.