After looking for missing calendar entries I found out that sync stopped a few days ago. Even manual sync did not help. Both direction did not work anymore. Contacts of same account were still synced in both directions.
As I could not find out the cause I de-installed app, therefore deleted all calendars/entries. Then I reinstalled App, added all calendars and sync worked again. But only for first time or maybe stopped after some syncs. As before I did not find something useful in logs.
I then realized that stop of sync nearly matched date of update to 3.3.8. Yesterday I did above procedure again but did not install newest version. I took 3.3.7. Now all seems to be working again.
Nextcloud contacts app has version 2.1.3.
As there’s not much more information I can give (found nothing in logs) I know it is not easy to help. So I’m just wondering if others are having same issues.