Hello Douwe Drost
Welcome to Microsoft Q&A Platform, thanks for posting your query here.
Based on the issue description, looks like you are having trouble creating a VM in a host pool with an image of a VM from that pool. When you create a VM in a host pool with an image of a VM from that pool, there are a few things you should be aware of to ensure that the VM is properly joined to the domain and added to the pool.
First, make sure that the VM image you are using to create the new VM is sysprepped and generalized. This ensures that the new VM is properly configured to join the domain and be added to the pool.
Second, when you create the new VM, make sure that you are using the same virtual network and subnet as the original VM. This ensures that the new VM is properly configured to communicate with the domain and other VMs in the pool.
Finally, when you create the new VM, make sure that you are using the same domain join settings as the original VM. This ensures that the new VM is properly joined to the domain and added to the pool.
Hope this helps.
If the suggested response helped you resolve your issue, please 'Accept as answer', so that it can help others in the community looking for help on similar topics.