The serial number has been fixed by using drscl. Thank you！
If dis- and reconnecting the board does not help, there is the (small) chance that the serial number got erased in the board. You can re-set it with the "drscl" command line tool:
Found DRS4 board 0 on USB, serial #0, firmware revision 21305
B0> serial 2172
I'm using 3 DRS boards simultaneously and their serial numbers are 2169, 2170, 2172 respectively.
Recently however, I obtain serial number "0" by DRSBoard::GetBoardSerialNumber() for #2172 board.
Data taking can be done without any problems, but I'd like to know what is happening.