Urbackup client hang forever while "Connecting for filelist (async)"

Hi, I use Urbackup Server 2.5.27 on Slackware64 15.0 with EXT4 filesystem and have a problem with a Window Server 2016 Urbackup client 2.5.24 with NTFS filesystem.
Server and Client are connected in multiple networks: both have 3 nic configured and can communicate through a switch.
The Client starts an incremental file backup each morning at 7:00 and it works.
When the Client start an incremental file backup during the day (they are scheduled at 10:00, 12:00, 16:00) the backup never ends and in the live log I see “Connecting for filelist (async)…”.

I tried to start Client in debug mode some days ago and this is the result.
urbackup_client_debuglog.txt (207.6 KB)
After some hours I have to stop backup from Server web console.

Please note that I have many other Clients (Windows and Linux) that works fine.
I already tried to delete Client and purge Server, uninstall Client then restart and install again, but nothing changed.

Could someone help me?

Thanks