Thank you for your reply Adam.
I double checked that all plugins are up-to-date. SonarCFamily 6.3(build 11371).
The scanner is the most current available, 4.0.0.1744-windows.
The problem persists.
I noticed that there is a sonar-scanner-debug.bat, so I tried executing that instead of the sonar-scanner.bat, but it seems to hang here:
Listening for transport dt_socket at address: 8000