gnus-calendar fails to capture to org file

Sending a reply works fine. Accepting an invitation opens buffers for
all agenda files. I also have a *CAL* buffer filled with event data, but
nothing is written to the org file. I configured the function according
to the comments in gnus-icalendar.el.

I used to use Mu4e and the capture worked fine. After switching to Gnus
I lost this function.

Unfortunately gnus-verbose=10 *Messages* does not log any errors.  I've
exhausted all my ideas. How can I diagnose the problem?

-- 
Bregds