In some cases you might have a contact list that only can be exported in the vcard 2.1 version (*.vcf files). There are a lot of old programs, in my case it was an old android device.
I just noticed that Nextcloud is not able to import any of the contacts (no specific error was shown: Show more details about the failing reason of an import · Issue #990 · nextcloud/contacts · GitHub), and after some investigation, I found out that you need at least version 3.0. There is an open feature request to do this conversion automatically:
https://github.com/nextcloud/contacts/issues/492
I didn’t implement this feature, but I found a work-around. Somebody wrote a small script in python:
For the list of contacts that I had, I didn’t see any issues. It would be nice to have this integrated but until then, there is at least some kind of workaround.