Sorry about that. We discovered a glitch in the original 5.6r7 release a few hours after it was published. As it would only affect VS2008 users, we simply replaced the original installer with a fixed version. Other than the VS2008-specific fix, both installers are identical, so we reused the same revision number.
If you are seeing this message, VisualGDB has downloaded the package catalog after the release, but before the fix. As a workaround, you can download the new VisualGDB update manually here. Other packages are not affected by this, so the error can be safely ignored.