Windows sees USB blaster but Quartus doesn't, why?

Sorry, if I re-read the whole thread, I don't clearly see that you made a test with different PC and same hardware. Good that

clarified this point.

If so, it's most likely a pure software problem. Are you running special kernel drivers like Wireshark USB capture or any other software that might affect USB operation. Also did you verify that Quartus 20.1 is using the recent JTAG server instance, can be checked in Services under Altera JTAG server or by executing jtagconfig in cmd window.

I would try a fresh install of recent Quartus standalone programmer which includes Signaltap and load your present Signaltap file from there.

Presume you are using genuine USB Blaster (Intel/Altera/Terasic).