I created another form from scratch and it worked as you said so the culprit appears to be custom fields. How can I make the form work with custom fields?
After refreshing and clearing cache a few times, the changes to the form finally cascaded to the website — so “a” seems to be ok now.
Regarding “b” — to test the integrity of the form, I enabled the Local List option and disabled the GetResponse integration. The submission was successful. So it seems now to point at something not connecting properly with GetResponse.
Thoughts?