Desolee, mais mon Francais est tres limite.
There is an issue with the Nextcloud client under Android. It consistently says "Synchronizaton failed". Last successful synchronization was 2017-12-13 07:13. I suspect that the problem is on your side since:
The problem occurred on both my Android devices pretty the same time. Both devices' Nextcloud clients have been updated to version 2.0.1 in late October. Since then nothing has been changed with the setup (as far as I'm aware of).
I signed up with another Nextcloud provider (ocloud.de). Syncing with their server worked flawlessly.
On one of my Android devices I reinstalled tzhe Nextcloud client from scratch and tried to set up the account for cloud.unixcorn.org. To no avail.
So all in all my conclusion is that there is an issue with the new servers in Falkenstein. From my professional experience with client server communication in general my suspicion is TLS handshake - although that's just a wild guess.
In case I can provide further information for investigation - just let me know.
Thanks in advance