Hyper-V live migration not pssible, Gen 1 IDE Drive the reason? (Incompatibilities : {21026, 40010})

Question

Hello dear community,

I am struggling to get machines to our new Hyper-V-Servers. A few months ago I have dont ehe exact same things on our first site and it worked flawlessly (~60 servers), but this time I can not migrate most of the machines.

Our setup:
4 old Standalone Hyper-V-Servers, 2016, fully patched
4 new Clustered Hyper-V-Servers, 2016, fully patched
10 or so guests per old HV Server that we want to migrate

I have removed all possible locally saved ISO files for the virtual guest's CD drives and matched all virtual switch names. So these issues have been prevented. Sadly I keep recieving incompatibility issues which all claim tp come from the path of the .vhdx files, which seems to make no sense because that's the whole point of moving machines, right? Move them from OldServer.D:\VMs to NewServer.C:\VMs (or so).

Now what I am trying to do is to live migrate machines from old to new servers (yes they trust each other and all use the same encryption setting). I tried GUI and PowerShell, which both worked flawlessly on our first site.

GUI
The GUI migration just stops and tells me the new machine is incompatible with the new host

PowerShell

  PS C:\Windows\system32> Compare-VM -Name guest1 -DestinationHost hostnew

    VM                 : VirtualMachine (Name = 'guest1') [Id = 'xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx']
    OperationType      : MoveVirtualMachine
    Destination        : hostnew2
    Incompatibilities  : {21026, 40010}


PS C:\Windows\system32> $report = Compare-VM -Name guest1 -DestinationHost hostnew
PS C:\Windows\system32> $report.Incompatibilities.Source | Format-Table

VMName   ControllerType ControllerNumber ControllerLocation DiskNumber Path
------   -------------- ---------------- ------------------ ---------- ----
guest1 IDE            0                0                             D:\VMs\Virtual Hard Drives\guest1\disk-0.vhdx

The issues are not limited to multi-drive guests but also single-HDD-guest machines have it.

Strangely a very few machines were able to migrate but the majority fails. Could it be the fact that many are still Gen 1? They have been created by a few older Admins, they always used Gen 1.

I am thankful for any useful tip, anything that helps us to not shut down every guest and recreate it on the new cluster.