fastriker i had the same issue with this box, mine was Leelbox Mxq Pro 4k, it seems that some of this crap box sellers use a different uboot version to block multi boot, i finally fixed it with flashing this update
Download Android Lollipop 5.1.1 stock firmware for MXQ Pro TV Box - China Gadgets Reviews
download it extract everything to an USB Stick which you then plug in to USB Port 4, then in Android use the Update & Backup app and select p201-ota-20160331.zip to flash it, take note it could be that when it is in recovery that it writes out an error, if that happens choose the p201-ota-20160331.zip again in recovery, then after flashing boot logo changed, which seems to be the real stock firmware for this device, after that i finally was able to use libreelec from sdcard and also was able to flash it on the nand.