The boards we shipped to you uses UART5 for debugging/deploying/loading firmware. We have changed it now to UART1 on SC20100 but kept is on UART5 on SC20260.
This means, starting preview 5, if you use MOD pin to select serial debugging, you need UART1 on SC20100, nothing changed on SC20260.
Here is the trap (IMPORTANT): while preview 5 will update the firmware to use UART1 on SC20100, the bootloader on your boards will still be the old one and it will use UART5. If you are confused, just do NOT use UART for updating firmware. USB still works as usual.