Sync contacts does not work on Android

Sync contacts does not work on Android 6 (Sony Xperia M5), latest client. Each attempt ends with “backup failed”.

How are you syncing contacts?

I go to Settings on the mobile client on my Android phone, then enable contacts backup and click on “click to start contacts backup”. I select a library to backup the contacts into. A gauge appears and fills up to 100%, then a message is displayed “Backup failed” and that’s all.

I never saw such a message when uploading with the seafile app. Does the backup to your local sd card work? And afterwards uploading from the sd card to seafile?

Yes, backup of contacts to SD card and upload of the VCF file to Seafile server works as expected.
I am using Seafile server CE 6.0.9 and 2.1.11 Android client.