Version 1.3 working with ("pip3 install pyqt5" and "pip3 install qdarkstyle") before I updated to 1.5.1 for fist time. So it's not 64 bit problem. Exactly after updating from 1.3 to 1.5.1 fluffy.pyw starts crashing for me.
I found a thing, if I remove "pip3 uninstall qdarkstyle", versions 1.3...