I can't - the "Add plugin" page doesn't even list the Cuberite organization for me.
Perhaps something went wrong while you were granting permissions?
Doesn't seem like it, the permissions seem in-place. Or I don't know what to check exactly

I found the application under Settings -> OAuth applications. Is the Cuberite repository listed there?
Also, I already added Core to the plugin repo, but left Coiny so you can test it out.
Yes, I've got it there as well, but nowhere else.
In that case I'm out of ideas. do you know what's going on?
I noticed, that it would probably be better to set a fixed height for each of the <a> tags containing one plugin and make the panels of the same height. (maybe cut text with …). That would make the site look a lot better i think
Yeah, I agree. But the overal look could should be changed if you'd ask me.
Also, I recognized that the repository loads very slowly. Perhaps we should move it to a separate server.
Part of the issue is the excessive GitHub API calls although that has been mitigated slightly with caching, although the inability to add a CNAME as it is hosted on azure is another issue.