I've resolved the issue and now I'm able to serve https.

As you already know, the problem is always the same and it's between the chair and the table ;-)

As noted by @ori (thank you), the issue was that I was trying to load the key from my own namespace instead of none. Also I was missing to reboot the server in order to load the key from /cfg/$sysname/cpuserver through factotum.

Thanks again!