So yeah... Space Exploration changes a lot of items (including belts) to include in their collision mask layer 14 (his space layer).
The belt highlight is a temporary item that exists over the belt also on layer 14. So, when we try to place a belt, it colides with this temporary item at this layer.
Quick fix: edit belt-highlight.lua inside prototypes folder, and at line 21, change layer-14 to layer-15