Not 100% sure I’ve figured this completely out, but I have a work around so I’ll post it. Pretty sure this has to do with NTLM vs kerberos. I think it needs to go up now without a 100% locked down root cause because I see a couple dozen people out there posting the same issue and nobody wants to listen to them – everyone immediately assume it is DNS related, which it isn’t, but ceases to help after they tag it with that issue.
The problem occurs when you add a forest trust to a couple of domains, then try to add resources directory to an XP box (such as add a user to a local group – in my case when testing some ADMT scenarios for a customer). When you click the locations tab, some trusts show up and some don’t.
What I found was if I removed the forest trust and recreated it as an external trust instead, the XP box could then add resources from that domain. After, I recreated it as a forest trust – the reference was gone again in “locations” but the user from the other forest stuck so I didn’t care.
Anyway, again, not 100% sure what the deal is here and I don’t have time to lock it down today but if you run into that… well, try external and good luck!