I think it was simply your 0.8a exe that was at fault when creating proper files in %appdata% or maybe something else in registry.
As soon as I started MB through the debug build (which I unpacked where MB was installed), it made all the things it needed, fixed the contents my %appdata% dir and made my old 0.8a exe work too.
So even the old 0.8a exe now launches fine.
It even works when I remove the %appdata% build, suddenly old 0.8a can create all the files there for itself.
I have no idea what pipe was at fault, but running the debug exe seems to have unclogged it. I can only observe that the problem seems to happen when MB tries to update configs in the old %appdata% (with pre-0.8 contents), that's when it clogs.