Ok, I've solved my own problem. The problem was caused by blocking of inbound traffic in a router upstream from mine. My home configuration consists of an ADSL modem/router of which I'm just using the modem part; the Ethernet-out cable goes into the WAN port of another wireless router (a better wireless router than the modem/router supplied by the ISP). But, it turns out the ISP-supplied router was blocking all inbound traffic (the norm, of course), but it didn't have knowledge of the outbound traffic (why, I don't know) for which it needs to allow inbound traffic. Changing the firewall rules to allow it to pass all inbound traffic to my own router solved the problem. My own router has it's own firewall, but it seems to know which inbound traffic to properly allow.
Thanks for reading, sorry to have taken up your time, but perhaps this will help someone else.