If your Original Prusa XL fails to heat correctly one of the heatbed tiles, the printer will show the message "Heatbed tile no. #: Preheat error".
Error name: MB heater disconnected
Error code: #17256
The error message includes the number of the affected heatbed tile. The error might indicate an issue with the cable connecting the indicated heatbed tile with the modular bed electronics, or with the heatbed tile itself.
Check that the indicated heatbed cable is correctly connected with the modular bed electronics, and with the heatbed tile, and reseat the connection.
Check the heatbed tile for physical damage, for example, a scratch on any heatbed tile surface, or a cable in bad status on the bottom of a heatbed tile.
Swap the heatbed tile connection indicated on the error with another heatbed tile connector, and check if the indicated heatbed tile number in the error changes, or remains the same after the swap.
If the error changes to another heatbed tile number after the cables swap, then the cable is causing the error.
If the error is still the same as before, with the same error code, and on the same heatbed tile number after the swap, most likely the heatbed tile is causing the error.