Now that's weird. I can see the picture from post #6 now and although the spoiler from your last post is empty for me again, I'm pretty sure that your paths aren't the problem. Otherwise the circle-shaped 'button' wouldn't be there. Also, it can't be any missing comma or something like that as your syntax is the same as mine and avains. And it works for both of us. And for you as well if you play avains mod, right? You could check for xml code that overwrites your button when it is used as button (not as circle-shaped icon). I can't tell you where to start with your search as I never heard about a problem like that before. Try to use some fantasy - the overwriting code probably doesn't look like it overwrites something. If it's a unique art (not a UU), you might check the CIV4UnitArtStyleTypeInfos.xml, too, although it doesn't deal with buttons. I could also imagine anotherreason for this although it'll sound strange. From the code listing you've posted, it seems like your button is not in the same folder as your unit is. It seems like most modders don't know about this, but on old systems it can happen that the unit does not show up on the lower left corner once selected, although all paths are correct if the name and the path of the animation file differ from those of the nif(s). Could be, that your button suffers a similar problem. So you might try to move your button to the knights subdirectory, correct the xml and see what happens.
Also, you might add another unit to your mod and give it a button. Don't overwrite your knight, just have them be in both at once. Check if the other units button works properly. If it does, try to compare the code of the two units and see whether there is still a mistake in the knights code. That's kind of debugging for poor men, but I can't think of anything else you could do.