I've found that the new ghostscript binary 'gs' has new command line parameters that are incompatible with their previous version...
I used to initialize gs like this:
Cmd = "gs -sDEVICE=pngalpha -q -dNOPLATFONTS -dNOPAUSE -dGraphicsAlphaBits=2 -sOutputFile=- -",
But that no longer works since this flushing is done only when the process quits...
The correct command line is then:
Cmd = "gs -sDEVICE=pngalpha -q -dGraphicsAlphaBits=2 -sOutputFile=%stdout -dNOPROMPT",
Everything works fine now, but I've re-read the very long Ghostscript documentation, and the solution comes from this page.