I got it, but I'm not sure exactly why ;-) Taking a closer look I
saw that the cable casing was getting out of the shifter when
trying to go back down from the highest chainring. Instead of
the inner cable release, the actual housing was getting out of
the shifter. I guess it meant that the cable was kinda stuck
somewhere. Maybe the gooey winter stuff or something.
So I replaced the cable, adjusted a bit, and it shifts OK now.
I've put a good amount of teflon oil on the cable. The shifting
is a bit hesitant, but that's how far I got by this winter temp
in the garage with barely a small heater trying to warm up the
air. Not to mention the ground and cold feet. For all puposes it
shifts OK, so I could as well keep it like this until spring. Or
I might spend an extra 15 minutes later.
I did try it in some snow, though. Before that fix I was stuck
on the large chainring. Now, getting it all the way down to the
small chainring and the largest at the back makes it a lot of fun
in deep snow.