Saving a project to a name it did not have before - Save As, a first save, or a project that is written to a new location such as when it is taken from a server - produces a project file that cannot be opened afterwards. Opening it reports:

Project rootnode itself was not loaded from file: <name>.ctprj

The project itself is not lost: the referenced models and diagrams are still on disk, only the project file that ties them together is unusable and has to be rebuilt by hand.

Ordinary saves of a project that already exists under the same name are not affected, which is why this shows up when a project first gets its name.