Hi @Razielh,
I think that error may be the 2 TB VHD ceiling, and it should go away if the backup repository is in block mode.
The same thing came up in July on GitHub (https://github.com/vatesfr/xen-orchestra/issues/10193, a 6 TB qcow2 disk), where @Florent explained that a repository not in block mode writes each disk as one whole VHD file, which can't go past 2 TB.
The reporter made a new share on their NAS, added it as a remote with "Store backup as multiple data blocks instead of a whole VHD file" ticked, and the 6 TB VM backed up after that.
The docs say it too, though as a bullet in a list rather than as a hard limit: https://docs.xen-orchestra.com/backups-and-dr/backup-features-and-settings#remotes.
While you're in there, incremental backups of qcow2 disks also need NBD on the job, or every run falls back to a full one (https://docs.xen-orchestra.com/backups-and-dr/backup-types/incremental_backups#nbd-enabled-backups); I haven't seen your job settings, so ignore that if it's already on.
I'm not sure an existing remote can be switched to block mode in place, so a fresh share like in that issue seems the safer bet. Might also be worth a mention to @Team-XO-Backend that XO could say this up front instead of throwing a RangeError mid-transfer.
