Fix startup handshake on Windows

Occasionally, on Windows, starting backend fails during handshake (some token receive, while expecting empty token).

The cause appears to be, that somehow the child process' stdout does not arrive in the frontend.

Unfortunately, the problem is not 1000% reproducible, making debugging/testing harder. Seems to occur much more often on some machines than on others, seems to occur particularly often the first start after a new version was installed.