In our case, the root project has an invalid character, so we received an error for each project that read:
... is not a valid project or module key
When setting sonar.projectKey only resolved the root project, it seemed logical enough to try sonar.moduleKey based on this specific error message.
I did search for “moduleKey” in the github repository out of curiosity though because I was also surprised with it being virtually unmentioned in the actual documentation.