Hi Bruno, sorry to hear you're having trouble with this. More often than not when I see this error there is usually a DNS or other connectivity issue preventing Outlook from reaching the MAPI endpoint. A good way to check is to check the following items from the affected computer:
If any of the above fail you can try adjusting local DNS on the client machine and flushing caches before trying again. SSL errors will need to be corrected for any of the noted URLs above as well (tailoring for your customer domain.) Beyond that, installing something like Fiddler Web Debugger Classic and configuring it to decrypt HTTPS traffic before allowing it to monitor Outlook startup may also provide some clues here.
Kyle Kerst
Acting IT Manager
SmarterTools Inc.
www.smartertools.com