i had this issue, i added another plate and transferred project there…deleted problem plate, ok now it slices…any item i put there would not slice good luck
putting this here to help someone in the future
If you split objects and deleted bits to print multiple of something, highlight them all with ctrl-A and then right click and say Merge. This will allow you to slice it again.
My fix - I checked the plate, apparently there were extra ‘ghost’ objects which I had to check and then delete. It was fine after that. Best to check all your objects against the process 