If your DSR-500AC is Rev.A1 (please check the sticker at the bottom of the unit for Hardware Revision):
Please load INTERMIDIATE FIRMWARE "DSR-500AC_A1_FW3.11B001E_WW" first.
Then load COMMON FIRMWARE "DSR-500AC_Ax_FW3.12B101H_WW".

If your DSR-500AC is Rev.A2 or higher, you can load COMMON FIRMWARE "DSR-500AC_Ax_FW3.12B101H_WW".