I'm quite new at C2, but I have been working on my first game for a couple of days now, and I didn't have any plugins or behaviours from the site. Then I browsed around and downloaded all of them with the C2Repo by Rex.
Now, when I try to open my project, I get an error message: "Cannot find action by ID".
Hope there is a solution where I get to use the plugins and behaviours on my WIP.