Hi there,
you probably know that you can move certain objects on the ground by means of the cursor keys. That's a neat feature if you want to spare time moving parts of your factory.
Your buffer box is also movable but unfortunately it leaves two pieces of belts behind that cannot be removed anymore. You can't build anything on top of them so the place where they lie is forever occupied.
You should either make your boxes unmovable (which would be a pity), move the belt pieces along with the box or make them removable.
Aside from that, it's a cool mod that I have been using a lot lately. Thanks for the work!
Cheers!
//edit: To get around this issue I've temporarily made the loaders of the buffer box minable and selectable. A quick fix, but I guess it won't hurt to fix it permanently.