@fanuelsen I was having a similar problem just now with XCP-NG 8.3 LTS and the latest XO. I was unable to migrate an MCS-created VM using XO (I was doing a Host Migrate within the pool only; no storage migration). Oddly, I was able to do the Host Migration using XCP-NG Center.
This was in a production Pool that I had recently set up, and this time I had been given a dedicated 10Gbps team with VLANs for migration and host management, and two separate 10Gbps teams for storage and for VMs, respectively.
To get live migration of my MCS-created VMs to work, I had to delete the Default Migration Network on the Pool's Advanced tab. I don't see a downside to doing this as all my NICs are 10Gbps, so all NICs should operate at roughly the same speed.
ETA: With the Default Migration Network deleted, I have confirmed that migration traffic defaults to going over the management NICs where it is desired rather than going over the storage or VM NICs.