DavX can't update Conctacts App

I have Nextcloud and DAVx5 installed on a few phones, all of which are signed into my self-hosted Nextcloud server. I am able to sync two-way changes to the calendar (changes made on any of the phones or Calendar app show up in the others), but for Contacts, it’s only possible to sync one way, from the Contacts app to any of the phones. If I create a new contact in the Contacts app on Nextcloud, that change will be pushed to all my phones, but if I create a new contact on any of my phones, those changes won’t be pushed to the Contacts app on Nextcloud, and therefore not to any of the other phones either. I am unsure what I am doing wrong.

Do you use a specific version?
When you create new contacts, do you see any corresponding entries in the logs (e.g. that the client tries to submit the changes and it fails, or does it not even try to connect to the server)?

I experienced something like this. The problem was years of different phones with a range of custom formats used in the contacts… The solution was to save the contacts on the phone and Nextcloud to useable files for later consultation. Then, trash all the contacts on the phone and Nextcloud. After that, rebuild the contacts list on Nextcloud and make sure they sync to the phone. Then put a contact on the phone and see if it syncs to the Nextcloud server.