Cannot convert Template to Virtual Machine

When I wanted to convert an existing template back to a Virtual Machine this entry was greyed out. Also the entry for “remove from inventory” was greyed out.

Other VMs can be converted to templates and back. I found a Community thread were a User deployed a new VM from the template which can’t be converted and after that everything was fine again.
I have done the same an lo and behold after the deployment I can convert the template again to a VM.
But do I have to do this with every template? If you can restart the vCenter Server services than do this and the other templates will be fixed. If you can’t restart the service you have to deploy a VM for every template you cannot convert back to a VM.