The latest version from Red Gate is 6.1. However the 5.1 version cannot automatically update to version 6 because there were changes to the Terms of Service, so instead you are redirected to the site to download the 6.1 version. This is mostly because of legal reasons as you can check in the following post:
Oi! What's going on with the .NET Reflector update mechanism?
After you manually update to 6.1 you will no longer experience any problems.