Error: After Effects was unable to find all of the media assets associated with this team project."

I've created a new team project, created a new comp and dragged an image from a library into the composition. The image appears, but if I close and reopen the project, they are gone. I also get a weird message about the media associated with it. 

If I add a file, it ap pears, but it will disappear after reopening the project:

If I open the suggested tab, none of the options are available:

I've already tried everything, including completely uninstalling Creative Cloud.