I believe that you are clicking on a link that is broken (maybe because it is too big), which makes the code invalid. Please make sure you are loading exactly the same URL that arrived in the email and not just part of it. For example, the line in the email can be broken in two parts:
http://in.nabble.com/user/Unauthorized$Authorize.jtp?code=YWRtaW5AY29kZWNvbWljcy5jb218Mjk0NzF8eXw0MTM5MjgzOTc=
If your email client ignores the rest of the code (second line above), then the code becomes invalid.
Please check that and let us know if you still have problems.
Regards,
Hugo Teixeira
Nabble.com