I HAD (past) the same problem, just with Cubase 9.5 AI, which was part of a bundle when I bought a Yamaha AG06 AUdio Interface.
I went thru all of the scenarios described above, including installing and removing all kind of things, first in - last out, and vice versa plus, I don’t know how many variations. ending up the same way like you guys did.
As a final result (again, and again considering the fact that Cubase crashes while loading its VST (!) System), I completely removed, then installed the “Yamaha Steinberg ASIO Audio Driver”. I found out only when looking out for alternatives, like FL Studio, and so on, where I “made” these applications crashing when attempting to adjust the respective application’s sound setup setting to use this ASIO driver (shown as installed and properly working in Windows Control Panel).
So I thought I caught a glimpse, removed this driver (Yamaha Steinberg ASIO), installed (NOT RE-installed) - all fine now, Cubase working. It is starting straight and fast without a single error message (not even those I had before, like “no input1 defined”, and bla bla). Result of days of guessing around, finally with a lucky outcome…
Maybe as a woman I am just a lucky girl, but maybe you guys can help yourself when checking your MAIN ASIO driver (which isn’t necessarily “Yamaha Steinberg” but most probably one with the name “Steinberg” mentioned). I wish so much that’d solve your problem, too, cos I know damn good about the frustration…
Last but not least, my setup: i7 7770 k, 16 Gig RAM, WIn 10 Pro, Cubase AI 9.5.21).
I hope this helps, all the best from Berlin,
Fleur.