i'm using GMS 2.1 since a long time, syncing contacts and calenders successfully. But since a few days there is a strange behavior.
A user (the only one on this system) has multiple adressbooks, two of them (the users default adressbook and one additional, named "XYZ" here) should be synced to an Android 7.1 device. Mostly, the user connects to the GMS only when in the office, and starts syncing manually.
A few days ago, it appeared that all the contacts and appointments have been lost on the Android device. Starting sync on the device, it synced the users adressbook into "Contacts", but not complete, for instance 485 out of 550 contacts, and 1 contact from "XYZ". Starting the sync again, both books are emptied on the device, and "XYZ" is synced, but also incomplete, for instance 520 out of 600 contacts. Within the next sync the books are emptied again, and "Contacts" is synced again. As long as the device is connected, this happens in a loop.
I've checked the nightly maintenance of the database, but it had no errors.

Does someone have a tipp, what to check?