@Eloth First of all, thanks for digging down the right topic! Unfortunately, my previous post missed one important limitation of editing tilesets in general, which is that they can't be external tilesets. Could it be that you're trying to modify an external tileset?
The UI could give better feedback in this case, but rather than adding this my plan has always been to lift this limitation instead. In the development snapshots, this has already been done, so I can finally stop explaining this once Tiled 1.0 is out.
In the meantime, either install a snapshot build (my preferred option, since you may be able to provide some feedback) or temporarily "import" the tileset so that you can edit it, and then re-export it afterwards.