I was playing US2 with only my custum bodies in, and then when i tried to place something nothing showed up, when i closed out the add menu, it just flashes out, abut then goes right back in, i couldnt place anything, i couldnt edit the menu i was on, as it would just flash a change then go directly back, i have no idea how this bug happened, so i cant really tell you how to replicate it, just put a bunch of custum bodies in, i guess. I honestly really hate this weird bug, despite it only happening to me once, tho i do think its a memory issue,
good luck fixing it