I am using the Contacts app on armored.net which is a free nextcloud provider. I installed DAVx5 on my Android phone, I exported my contacts from my phone to a vcf file, deleted my contacts from my contacts app, imported the vcf file to my DAVx5 contacts which is my armored.net Nextcloud account. Then I went to this URL to download my contacts
And then I deleted the contacts from my armored.net account and tried to import them again from the recently download export file and it will not work. I even tried to import the recently download export file to my phone contacts and it not work. The error message says “Failed to import”
Unfortunately you’re not providing any Nextcloud version information. I could be possible that a problem appeared which has already been addressed or fixed in the latest version. It might also possible that some Tags are malformed, due to a problem in the Android client app.
Nextcloud accepts such data and runs syntax checks as soon as you access such an entry using the web gui. In this case Nextcloud shows a warning and ask you to double-check and confirm that all data is ok.
To find the root cause of the problem I would first recommend to check the Nextcloud log file for any error on the import attempt.