When you create two projects with the same name, but with different case letter (eg. project and PROJECT), you may not be able to see both in the Open/Join Teamwork Project window, therefore you can’t Join the project.
In general, ARCHICAD and BIMcloud/BIM Server are case sensitive, so you are able to create projects with the same name, but different case letters. Unfortunately this is not the case with the Open/Join window. If you create multiple projects with the same name, You may not be able to reach them (except for a single one).
The Solution is simple. Rename the projects in the BIMcloud/BIM Server Manager, and avoid giving multiple projects the same name in the future.