For dq250 checksum doesn't matter.
I think you interrupt the writing or you flash it with other firmware from other dq250 mechatronic model. For first option you can recover in boot mode , for second....throw it to the trash.
Not correct info
Exx, Fxx and MQB can always be recovered in boot if they were flashed by OBD.
Interrupting flashing - nothing happens. ECU goes to bootloader.
In his situation the checksum is correct, but the content of the OBD flashable ECU areas is not.
It tries to boot but crashes over and over again, in this case possible only boot recovery. But for CXX there are no boot protocols in any tool, so that one goes to trash.