The IP entry is irrelevant. That's only recorded if you join a LAN server.
Assuming you've checked your LOCAL firewalls (i.e. if you used the Windows firewall, remove LFS from it, then run LFS in windowed mode, and reallow LFS access when it prompts you again - use roughly the same procedure for other firewall software) - the reason for this is how firewall software identifies a program and whether or allow it access, I won't bore you with the details.
If it's not this, then I'd suggest talking to your ISP as to whether or not they changed any internal policies, such as packet mangling (throttling, blocking, etc.) before I started looking at anything else local to your machine. Given that only a small minority of people are having issues it makes more sense that it's not something related directly with LFS itself, but perhaps some combination of hardware and LFS, or simply your ISP changed something when the patch was released.
Quickest way to test that would be to download an older patch and try to join an older server. If this works it's still no guarantee that things have not altered at your ISP. IT could be that they used a way to detect LFS and exclude it from any packet mangling, and that now no longer works (unlikely as I do not believe the netcode has changed much in Z).
If that works start providing more details about your computer, turn network debugging on in LFS, and attach the debug to a post when you fail to connect.
The fact that you can talk to the master server is irrelevant, as thats only used to get a list of servers. Since you can get a list, you clearly can talk to the master.