Cannot load project (CMake-3.17)

After a system upgrade from Ubuntu bionic to focal, I can no-longer load my large project.

CMake runs fine but QtCreator reports (on the command line only; the GUI does not present useful diagnostic messages):

qtc.cmake.fileApi: Directory "src/addons/vortex/extensions/openfoam/schwarzCoupled3" : project index 155 is broken.
"Invalid codemodel file generated by CMake: Broken indexes in directories, projects, or targets."

If I remove the subdirectory mentioned in the message, new errors occur at other subdirectories of the project.

Before the upgrade, I used the same QtCreator version but with CMake-3.10.

Other smaller projects work.

My impression is, that it has to do with the CMake version

Is there a way to get more helpful diagnostic output?