Pydio pro on android, automatic backup always fails


I cannot use the automatic backup feature from the Pydio Pro Android app because it backups 20 files and then failed “La dernière tentative a échoué:”.
There is no error message.
I use the last version (march 22th, 2017) of the app and the last version (8.0.2-1) of Pydio.

I use a Pydio server on linux. Everything else work correctly (upload, download, browsing…).

Thank you.