The available SR for a VDI migration seems to be dependent on the view in XOA5.
In the VM-view (home -> VMs). I can migrate the VM including the VDI as usual - Hosts and all SRs are shown. If i select the "Disk" Tab however, the "Migratie VDI" just show the SR the VDI is currently on.
If i get into the Storage-View (home -> Storage") first, then open the "Disk" Tab the "Migrate VDI" again showing all SRs.

To me it looks like the Selection of SRs in the VMs-View / Disk-Tab is broken, when trying to migrate a VDI.