Connexion Error -55 on server side and FileSrv: Recv Error in RecvMessage on client side

Hello,
I face difficulties to set up my urbackup server.
I have server ubuntu 20.04 with urbackup from ppa:uroni/urbackup
I try to connect 2 kind of clients : ubuntu and windows. none works

For ubuntu client and server
I face that message in logs:
Connexion Error -55 on server side
FileSrv: Recv Error in RecvMessage on client side
I check my open ports :
server → 55414,55415 TCP and 35623 UDP
client ubuntu → 35621,35623 TCP and 35622 UDP
I see in forum that could come from write permission in /var/urbackup on server side : I check
#sudo -u urbackup touch /var/urbackup/toto works well.
On client side, urbackupclient is running as root : I assume no possible file permission issue.
I reboot both client and server…

I am very disappointed because I had previously an urbackup conf that work so well !
I wanted to improve a bit my server upgrading it.

I reinstall everything on my server (by script : it should be identical install)
I uninstall and reinstall my client.

Any idea where to look at ?