I recently replaced a toner cartridge, and now my Canon 5100 is showing a 5100 error. Is there a connection?

While unlikely, it's possible the toner replacement triggered the error if the new cartridge wasn't properly seated or if it caused a slight mechanical shift within the printer. Try reseating the toner cartridge firmly. If the error persists, it's likely unrelated to the toner and points to the LSU/laser block.