That would seem to rule out name lookup, then.

What do the samba.conf files look like on the "broken" servers? What's the difference between those and the working server?

What are the permissions on the files?

IIRC, access via mapped drive and unc path use different authentication schemes. Is it possible that it's the authentication step that's failing? Are the samba hosts passing authentication back to one of the Windows machines, and have an outdated Kerberos ticket? Have the IP addresses of any of your machines changed, or are they statically assigned?

Though, since you can see the server content, just not open it, the above seems like it's not the solution. I don't know how fine-grained the UAC on Windows are. Maybe it's only getting hung up on "open-for-write"?