I'm getting the same error on my corexy.
when I try to downgrade to 1.21 rc2 I get
M997 S0 Error: M997: Firmware binary "Duet2CombinedFirmware.bin" not found
I managed to restore RC2 after hitting erase and flashing it using SAM-BA.
Working normally now, but still using wifiserver rc3, DWC rc3
Maybe try renaming the old firmware that you are trying to downgrade to "Duet2CombinedFirmware.bin". The newer versions look for that file name, the same way you had to rename the Duet2CombinedFirmware.bin to DuetWiFiFirmware.bin when upgrading the first time.