OK, I followed the instruction and it has been worked for 3 month - but now I get: Unable to update PLSK.xxxxxxx: an error occurred while processing your key and login to plesk say: no licence
In that case it's best to ask your provider if the license is still valid for your new server. Your provider is the license owner, they can tell if the license is still valid.
A license is bound to an IP address. If your new server has a different IP address than your old server the license has to transferred to the new IP (or you need a new license).