Hi Akja,
Sounds like a networking problem to me - if the warning only appears when the machine is connected to a specific network. Can you still connect to the machine regardless of the warning (I've had that before)?
Also, is the home network NAT'ed outwards? If so, then does your modem / router employ some sort of firewall? How about your home internet provider? I'm not sure what UMTS is, but does it provide a specific machine a direct connection to the internet?
I'd also go ahead and collect logs in both situations (with and without warning, on the target machine) and
submit a bug.
Cheers,
Oren