@jokurinopoloski are your USB NICs running on VLANs? If so, you could check /var/log/kern.log for xenbr2: dropped over-mtu packet: 1502 > 1500 [...] messages. Maybe not related, but I had a problem with connecting to a NFS SR. While ping and SSH worked, connecting XO from source to the NFS SR failed. After changing the USB NICs MTU from 1500 to 1502, it did work.