In multi-root workspace environments, a project debug launch from the Explorer (via right-click and selecting Debug Project) can fail with a "launch.json is not a valid JSON file" error.

It looks like the launch configuration for the project is added to the .theia-workspace file and not the launch.json. When checking the list of available launches in the Debug view, multiple entries for the project will appear, one for each folder.

Note that the project only exists in one folder. It does not exist in the other folders.

Interestingly, trying to run any of the entries will successfully launch a debug session.

Eventually it can get to a state where all launches are cleared and all open folders in the workspace are closed. The .theia-workspace folder also gets cleared. It is unknown what exactly triggers this but playing around with launches and opening the various launch.json and .theia-workspace file will eventually trigger this.

NOTE: The project debug launch from the Explorer will work the first time (and create the launch entries in the Debug view). It is subsequent launches from the Explorer that fail with the error.