I assume the error message would also give you an element ID. Do a find and select in the 3d window or separately on each floorplan for that particular ID and see what is highlighted.
Note that it is case sensitive and recognises spaces, so "SLA - 002" may find it, but "sla-002" would not. You may find more than one element with the same ID, but try deleting the most complex first and see if that stops the error. If not, undo and try the next.
As Eduardo says, also use the marquee to narrow down the area if there are lots to choose from.