VM migrate in partially upgraded pool (7.0-7.5) not working due to method VM.assert_can_migrate_sender

Right now the state of the pool is "partially upgraded to XenServer 7.5". 

• I cleared the pool-master of all VMs (live migrate to pool-member).
• Shut down the master and started the upgrade to 7.5 via base-image iso.
• Upgrade without any problems. 
• Server comes back online again, now the pool is in state "partially upgraded".
• Start to migrate all VMs from the pool-member to the now upgraded pool-master.
• One VM on NFS shared storage migrated live and running without problems.
• First try to migrate a running VM that is on local storage fails with:

Migrate to Server: You tried to call a method that does not exist. The method name that you used is VM.assert_can_migrate_sender. 

And that's the issue. I can't migrate the VMs to the upgraded master to finnish the pool upgrade by emptying the pool-member still on 7.0.

Tried to move an offline VM on local storage to the master and this breaks with error: INVALID_VALUE

Found this (via: https://github.com/xapi-project/xen-api/blob/master/ocaml/xapi/xapi_vm_migrate.ml )

let assert_can_migrate_sender ~__context ~vm ~dest ~live ~vdi_map ~vif_map ~vgpu_map ~options =
 (* Check that the destination host has compatible pGPUs -- if needed *)
 let remote = remote_of_dest dest in
 let remote_for_migration_type =
 match migration_type ~__context ~remote with
 | `intra_pool -> None
 | `cross_pool -> Some (remote.rpc, remote.session)
 in
 (* We only need to check compatibility for "live" vGPUs *)
 if Db.VM.get_power_state ~__context ~self:vm <> `Halted then
 Xapi_pgpu_helpers.assert_destination_has_pgpu_compatible_with_vm ~__context
 ~vm ~vgpu_map ~host:remote.dest_host ?remote:remote_for_migration_type ()

pGPUs are the same

I've opened a forum discussion but so far no clues on what's happening (granted it's been just few hours and I'm in Euro timezone)
https://discussions.citrix.com/topic/397948-upgrade-pool-70-to-75-xenmotion-storage-not-working-method-vmassert_can_migrate_sender/

Any help much appriciated. If any infos are missing I'm happy to add them..