That error message usually means an error in the config format. Send me your config.toml via DM and I'll have a look.
Discussion
I went through the code source and found it. The problem was that I had not set the "terms_message =". It works now.
But now next step is trying to wrap my head around how this actually works. So far I found out that most clients don't support this directly (or is there some trick?) So I guess I will have to make a webpage for a user to pay for access. Or it there some smoother way for a nicer user experience?