HercGui createprocess error on perfectly well functioning Hercules

I have a perfectly well functioning Hercules environment in command prompt mode.  I now wanted to add Fish's GUI (the 64 bit version).

However, as soon as I try to invoke it, I get

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x:/Hercules/Prometheus.exe -fX:/Hercules/mvs/mvs.cnf -d =l dyngui

failed with rc=2 (0x00000002) The system cannot find the file specified.
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I don't know why it tries to invoke Prometheus instead of Hercules.exe.  I tried to look if a 64 bit version was called Prometheus but was unable to confirm.

Any help would be greatly appreciated.