So, for a while I've experienced this problem that didn't previously occur a year or so ago - when trying to build near the edge of the plot, you'll be told you cannot build if you go over the border. This is especially a problem when trying to build with little space between the border and a wall.
You have to be standing inside the plot in order to place a block if you stand outside your plot it won't work
It should not be happening as far as I can tell from the code. Does it happen when you stand outside of the plot, or when you click on the slab?
I know why it happens. The plotme plugin thinks that you're building outside of the plot. This is because you're placing the block while pointing at a block that is outside the plot. I'm really not sure how to do this (Since I don't do plugins) but the basic logic is that you should be able to place blocks in the corners of the plot but not destroy any blocks in there.
So is your post. Anyways, what I do to get over this, is I fly up a bit above where I'm building, as this occurs on the edge outline of the plot, and place the block from there. It works if you place it that way, rather than facing the outside of your plot. You also have to be in the plot when you build from above.