Windows 7 HomeGroup simply doesn't work - no useful error message

I've got two Windows 7 Pro machines that I'm trying to connect using a HomeGroup (partly just to see if it's useful, since I'm already pretty comfortable with normal windows network sharing), but this new, "easy" sharing system simply isn't working. Nor is it giving me any useful error message.

I set up a HomeGroup on my primary system, and shared a directory or two to get it started. No problem.

I went to my living room PC and told it to join the HomeGroup. I very carefully typed the password in, and got an error saying something like "There was a problem connecting to the HomeGroup." That's it. "A problem." What kind of problem? Who the heck knows? I checked and re-checked the password, typed it in again, went back to the primary PC to verify I'd written it down correctly, etc.

A bit of info about my setup:

- Both systems are connected to the same-named workgroup. Both are definitely able to see each other on the network, since I'm already sharing directories on the network using the normal method. Aside from the fact that Win7 has a bug in it (that apparently has existed since Vista, based on a web search) where telling it to remember login credentials doesn't remember login credentials, that's working fine. (To hopefully solve that problem, I have manually created a certificate, but haven't turned the system on since then to see if it worked).

- My primary PC has a wired connection to the router that's connected to my cable modem. The living room PC has a wired connection to a second router that's connected to the first one via 802.11N connection in "Link Removed - Invalid URL," which basically just connects it right to the router, daisy chaining the two routers together to create one eight-port wired router.

- Both are Linksys routers, with the primary router being a WRT600N using the latest Linksys firmware, and the bridge router being a WRT150N using DD-WRT v24 preSP2 (Linksys' firmware doesn't offer a wireless bridge mode).

I looked around on the Internet, and found only two possibilities. One person claimed that HomeGroups required IPv6, which most home routers don't yet support, but others quickly corrected him, saying they've got HomeGroups on IPv4 networks. Another person pointed out that HomeGroups will fail to work if a system has a public Internet connection of any kind, but both are connected only to "Home Network" and "Local Area Connection." No secondary public connections.

I guess the most basic question is... where do I start? Everything seems to be working fine in terms of the network setup, but Windows gives me an absolutely worthless error when I try to set up a HomeGroup.