Problems with accounts that use two factor authorisation?

can't test yet as I'm still on 3.2.1. but using device specific passwords, as advised for 2FA, works fine for DAV. Why should that change for the new Nextcloud integration? if I understand it well, SFOS doesn't care or know about 2FA. If the password it gets works, syncing should too.

There was or is one issue with the DAV sync though (both calendar and contacts) that causes write operations to fail after a certain time. Maybe i can dig up the TJC thread it was described in.
For me until now that meant recreating the account every few months. Otherwise changes or new appointments/contacts won't be written anymore to the Nextcloud server. Is that the problem you're facing that's specific to enabled 2factor?

Oh yes, while I had TFA active I had the problem you described that calendar changes weren't synced. I thought TFA was responsible for that cause I just set that up shortly before. I deleted the account, but after after that I didn't work anymore at all (and yes, I used a new application password for that case) - no contact or calendar was synced. So I cancelled TFA - and got back my contacts and just two from eight of my calendars with calendar sync not working (these two calendars stayed empty).

With SFOS 3.3 I deleted the old Caldav/Carddav account - nextcloud seems to work well now, syncing goes on as expected both ways.

I'll give TFA a second try now with nextcloud. Thanx so far - and it seems my OneDrive problem has another reason.

What you describe makes me wonder if the problem with writing to the DAV server is maybe related to 2FA. in fact, I never change passwords but reuse the device specific ones I generated in Nextcloud when recreating the account on Sailfish.
as said, in general it works but after a few months it starts having hiccups. the time it takes varies though