Linux clients did not start backup

Hi everyone,

there a lot of topics already concerning linux clients, but non of them brings a solution for me. I have the following setup:

Urbackup Server as Docker (Version 2.4.13)
Windows Client (Version 2.4.10 working well with file and image backup)
3 Raspberry Clients (one with Ubuntu and 2 with Raspberry Pi OS Buster)

All 4 clients are shown in the server interface correctly, they were added automatically. The windows pc worked out of the box with every feature.

The 3 Raspberry clients are not working at all. If I start a file backup from the server (I know that image backup is not supported yet) it says, that the task is queued and after a few seconds its gone without any backup or log entries. If i start for example on the raspberry itself with “urbackupclientctl start -f” I got the message "Waiting for server to start backup… " and after a while I got the reply “Timeout while waiting for server to start backup”. But at the server there is not message at all.
Path for filebackup is set to “/home/” for testing purposes. I checked it with “urbackupclientctl list-backupdir”.

What am I doing wrong? Someone any hint for me?