I have seen a lot of postings on the Kayran QTE problems, but I suppose it does not hurt to post this again.

So, I have been fighting the Kayran, and at the end of the QTE sequence I was supposed to press the M0 button. Well, the mouse buttons start numbering at 1, so it was really puzzling as to what to press. And, as you know, experimenting takes too much time.

Well, the solution is almost too simple. On the default settings the QTE tells you to press SPACE. This is a default mapping for Dodge. I had my control scheme changed so that the Dodge action was mapped to M4, which on my mouse is the button I can press with my thumb. So either mapping Dodge back to SPACE or pressing the M4 at the right time did the trick.

I just wish I didn't have to explore for such things in an otherwise great game.

So, if I have this right - you changed the key mapping and are upset because the default key that showed onscreen did not change? It won't.

No, I approve that the on-screen hint showed a changed after I'd remapped that key. But the hint didn't show the key that I mapped, which is M4, but rather a key that does not exist, i.e., M0.