Add three more venues at Autumn Moon Festival (Hameln, Germany)

It (or I) did it again :-/

No worries, it's a pain in the neck. I fixed it.

it requests the dates of the festival from the user,

Yes, but now you know that you shouldn't enter the dates then and something went wrong :)

The web programming guys should have a look at this, I guess.

Your guess is correct. It's on the TODO list, but with a low priority to be honest.

Took me ages to find it!

Thanks for the feedback.

By the way: how to delete a setlist alltogether?

That's a moderator feature, post it to the Duplicate / Delete setlist topic in the website support forum.