Oracle Guardian JVM terminated exit code -1

I am trying to run Guardian on my 64bit Windows..... no matter which heap parameters I put in the guardian.ini file, it keeps failing on startup with this error:

I have really no clue how to proceed.

Even
guardianHeadless.cmd -g help

fails with the same error.

I have tried providing a 32bit JVM, to no avail.

Funnily, the Linux installation works without a glitch.