@florent Unfortunately it is not working. Yesterday when I checked it was actually backing up a VM which had only 2 disks and by mistake I thought it was one of the VMs with a lot of disks attached to it. Sorry for the confusion.

It still looks like this

Even though I added to my config file at /etc/xo-server/config.toml:

#=====================================================================

# Configuration for backups
[backups]
diskPerVmConcurrency = 2

Other settings I defined in /etc/xo-server/config.toml are working just fine. E.g. setting HTTP to HTTPS redirection, SSL certificate or similar.
So I think Xen Orchestra (XO from sources) reads my config file. It appears that the backup option "diskPerVmConcurrency" does not have any effect at all. No matter in what file I set it.

Is this setting working for anyone else?