Summarize for questions stopped working

I have Camptix set up with 6 tickets, three free and three paid (PayPal). Each ticket has 3 questions: Default with name and email, a checkbox with some 25-30 alternatives and a radiobutton with yes/no.

The summarize tool workes as expected for standard questions like Ticket type and Purchase date, but summarize for additional questions has stopped working. Selecting the question and then pressing Show Summary reveals just a white space. No PHP or JS error.

What could be wrong?

https://www.ads-software.com/plugins/camptix/