I have now conluded the problem is related to MCS (Machine Creation Services). MCS does not support XenMotion Storage. When you move vms with regular XenMotion it all works as expected. We are pretty new to MCS thats why this problem shoved up.
If you migrate VMs in XenOrchestra with the default settings without touching the migration network it works fine because its using XenMotion 
If you choose a migration network like the picture below it won't work because its using XenMotion Storage
Even if XenDesktop is not officially supported on XCP-ng it has been working fine for us. We have 80 terminal servers on 18 XCP-ng, been running it for years, no big problems.
Anyway thanks for your response.