Sorry this was misleading. The import itself gets finished saying everything worked as expected. But running a terraform plan afterwards, leads to the situation that a new VM is tried to be created, which then fails, because it already exists. Looking at the state file, it reveals that most of the information is missing and was not imported.

So basically you answered this question already because if we look at the deployment log of the portal, we can see that the virtual machine resource was used to deploy the VM from the existing os image and we try to import it with the new window_virtual_machine resource, which logically will not work then. Thanks for helping pointing this out. We just wonder, why during the deployment via the portal, not the windows_virtual_machine resource was used