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