Windows 10. No, I jumped from .0a1 to .0a3. Now I installed .0a4 and have the same problem. I'm afraid of going back to .0a2, was taught long ago to never downgrade anything. :-) But I have 3.5.1 installed separately, and its IDLE keeps working without problem.

My config-main.cfg is
-------------------------
[EditorWindow]
font-bold = False
font-size = 12
font = consolas
height = 25

[General]
autosave = 1

[Keys]
default = False
name = Veky2

[Theme]
name = IDLE New
------------------------

(Veky2 is what I've made after Veky stopped working, thinking that it got corrupted or something.) Yes, I know about regular semantics of name2, it's just that I was desperate. :-o

I tried running from the console (py -m idlelib, right?), and I don't see any error messages. It's just that every time I go to Options > Configure IDLE > Keys > Key Set, radiobutton Use a Custom Key Set is selected, and option IDLE Classic Windows is selected beside it. I can select Veky2 and click OK, but it doesn't switch, and the next time I go to the same place, it's IDLE Classic Windows again. :-(