The version of system.img should be the same with your phone's OS version, be sure to flash both boot.img and system.img with the same version
C:\> fastboot flash boot boot.img
C:\> fastboot flash system system.img
(Optional) Update modem image if you encounter communication problems.
C:\> fastboot flash modem FHD@modem@ZE600KL@8939@NON-HLOS-64bit.bin
* Return to stock ROM
1. Download stock ROM from official page: http://www.asus.com/Phone/ZenFon ... /HelpDesk_Download/
2. Grab 'recovery.img' from the same folder with preroot system.img
3. Restore stock recovery by the following command: 'fastboot flash recovery recovery.img'
4. Enter recovery by the following command: 'fastboot oem reboot-recovery'
5. Select 'apply update from ADB' and use the following command to restore stock ROM you've downloaded from above link: 'adb sideload UL-Z00T-WW-x.xx.xx.xxxx-user.zip'
Stuck at Fastboot mode after flashing ZE601KL_WW_21.40.1220.1698. Always goes to Fastboot mode after reboot. Upgrade from ZE601KL_WW_1.16.40.1338. Are you the original poster of the updates?
Finally got it to work. My ZE601kl is now on Marshmallow and rooted! Flashing the pre-root MM ROM over Lollipop OS will not work. You have to upgrade to MM first before flashing the pre-root image.