The firmware was the right one. I just removed region 0x66 (it's just a zip file with img_rsrc.gir and Langs directory) from the firmware.
Usually it helps to flash the firmware over USB when one have "unable to program the requested memory region", like [Only registered and activated users can see links. ].

I don't quite understand why you used GarminCure3. Your device was seen by the PC anyway.
Please tell me if your device is recognized by the PC as a flash drive now.
Also please tell me how exactly you flashed the firmware. Did you copy a GCD file somewhere on the device? Or did you flashed the firmware over USB with updater.exe or WebUpdater.exe? Can you enter the pre-boot mode again and flash the original firmware over USB? (use ORIGINAL option in GarminCure3).