Thanks for your report. Can you please provide verbose steps to reproduce and DAVdroid debug info?
I wish I could, I have no Idea how to re-introduce those faulty VCARDs back into the system (I presume they got there from earlier versions of nextcloud calendar).
If I were to guess the reproduction steps, thou would probably behave faulty (i.e. containing escaped comma) VCARDs on your server of choice SYNC (creating faulty entries on your android device) correct VCARDs on the server (i.e. remove comma) SYNC → wonder where your actual categories/groups went.
Why has the VCard version changed from 3 to 4?
As far as I can see, all VCARDS written by DAVdroid 1.3 and later are VERSION:4.0, so I do not fully understand the question.
Is DAVdroid still set to the CATEGORIES group method?
I don’t remember ever changing this setting (as far as I can tell your question was the first time I checked that deeply into settings). The chosen method is (and always has been) “Gruppen sind Kategorien der Kontakte” -> “groups are categories of contacts” (loosely translated from german translation)
I’ll have to be able to reproduce the problem so that I can have a look at it.
Yes, I can totally understand that. I will gladly help. There’s limits on what I can do, though. I don’t really have a debug setup, only my actual phone with my actual contacts.