Hello Christoph,
Thank you for reporting this bug.
I was able to reproduce the issue. I did not understand the reason though.
As a workaround for demo.openpetra.org, you can create a receipt just for one contact:
43011016 (Lang Erich)
Hmm, now it works for all contacts. Perhaps restarting the server did help...
Let me know if you run into the issue again!
All the best,
Timotheus