I feel really stupid, there were some new updates to be applied, and one of those solved the problem.
Thanks @danp for pointing me in the right direction.

@danp Thanks for your reply.
I reviewed the logs but found nothing I could use to explain why the clone/copy operation is failing. I´ll do some tests and post the content of the logs files here.

Hi

Last week I upgraded one of my pools from 8.1 to 8.2 with the recommended method.

Now I've found that I cannot create new VMs from any of my custom Windows templates or snapshots. Whenever I copy/clone one snapshots, VM or template I get the following error:

The server failed to handle your request, due to an internal error. The given message may give details useful for debugging the problem.
message: Subprocess exited with unexpected code 127; stdout = [ ]; stderr = [ ]

I've also tried to create a new template from a fresh Windows 2019 server with the same result: I've created a new VM from the base Windows 2019 template, installed the OS and XCP-ng tools, updates and then converted it to a template. Using this new template as a source for a new VM.

If I take a custom CentOS 7 template all the operations described above work flawlessly.

If you need more data please let me know.

Thanks a lot