Request sent to Orchestrator is invalid when I want to start automated execution from test Manager

I have a test set with a data driven test case in Test Manager (UiPath cloud). I also have an unattended robot configured. Now when I try to execute (automated) my test set, I get the toast message: Request sent to Orchestrator is invalid. (and I see a 400 in my chrome console).

What am I doing wrong? How is one supposed to run a test set using a local robot, from orchestrator?