When I download ships and install them to my savegame, the game tells me that all parts are "locked or invalid" when I want to launch. These crafts are built with nothing but stock parts. And I have the parts unlocked in the R&D tree, that's not the issue. I know that because I see a part on the list of invalid parts but at the same time I see that exact same part available on the left in the VAB.

I use KW Rocketry and the SpaceY mods but none of them replace vanilla parts, merely add new ones.

Can this be related to game version or something? I play on the latest version and most of the crafts are created on an earlier version, naturally. But I don't see how the same vanilla parts aren't recognised just because the version is different