The main intention was just to give feedback for other users who try to use iCal. Especially when setting up behind an Apache proxy.
I realized now, that ical is not the new implementation and the paths at https://forum.kopano.io/topic/1341/request-for-feedback-new-caldav-and-carddav-implementation-for-kopano are for the other version. I will perhaps test that version also.
A main problem for my first tries to get it working correctly was, that the ical implementation accepts ever context path and is answering on every context path partially correct, but only using the context path /caldav the answer is compleatly correct. This is missleading when trying to setup everything.
About subfolders: I created a folder “Test” inside the Tasks folder and tried both URLs /caldav/<username>/Test (like written in the manual) and /caldav/<username>/Tasks/Test and neither worked. But this is also nothing I need, I just mention it to document it.
So no need for any action on your side - just a report in case someone else is trying to setup a similar usecase.