You need to update the navcore on the device ... (App 7.903 will not read 845 map)
SE Navcore 9.101.516023 works fine on the 910 found here
You will lose auto switching between day / night, but the rest should be fine
Just make sure you keep the backup you have, also try the new navcore WITHOUT placing the "system" file (bootloader) on your unit. Always a good idea not to upgrade the bootloader unless you have to.