Client wont connect to server

Hi, it’s my first post here, mostly because urbackup works well without incident (21 webservers backed-up with it).
thanks for this great tool!

but today i’m facing an issue that i can’t resolved by my self.
I have a client that refuse to connect to the server, i followed the same procedure of installation as my other clients, i deactivated firwall on both client and server, i clean-reinstalled the client many times, but i can’t get it to connect.

the only message i have is on client side

Socket has error: 111

server is a dedicated server running debian buster with UB 2.4.12
client is proxmox CT running debian stretch with client

some help would greatly appreciated