Danke Andreas 
This might be a stupid question but you’re talking about that “interactive” mode? Do you mean the option between the forced way and the manual option? We’re actually using SCCM to deploy our updates and for this server, the software updates are deployed as “available”.
I’ve also checked this location : HKLM\Software\Policies\Microsoft\Windows\WindowsUpdate\AU and the key NoAutoUpdate is set to 1. As expected…
Out of curiousity, I’ve set this key to 0 and restarted the service and restarted the VBS : same error. Deleted the key, restarted the service, stopped the SCCM client (so it doesn’t overwrite this with its client options) : and alas, the same error in that VBS.