Hi armin, just an additional illustration using your model. Right now you're trying to fillet only these edges:



Which gives this result (I did the fillet on a 10 times scaled up model here which gets more accurate results) shown zoomed in to the corner:



Due to the various angles that those surfaces meet, that is the proper fillet result. They are closer to being the same angle than in my exaggerated example in my previous post, so they are closer to being the same in width than the other example, but it is the same thing.

What you may want to do instead is to select all of these edges:



Now when you fillet there will be the addition of corner patches where different fillets run into one another:



You'll usually want to have corner patches in between pieces, otherwise the fillet surfaces themselves will only tend to be perfectly aligned with one another in width if everything is touching each other at the same angles.

They will still make a valid solid if they are of different widths though, like the pieces are all trimmed and joined to one another for example (except in the one that you saw with the excess piece not getting trimmed properly, but I think that should go away if you scale up).

- Michael