This is reviving a rather old thread, but since I had the exact same issues as the thread creator and his error description is very good, I wanted to add my solution.
As a precursor, I used the toradex-imx7 community recipes as a base and rewrote the parts needed for the imx6ull board. For flashing I followed Mirza Krak’s description here: https://hub.mender.io/t/toradex-colibri-imx7-dual-512mb-colibri-imx7-solo-256mb/81
Afterwards the board booted up only once throwing a lot of ecc errors and from the second start it only went into u-boot.
On my search for a solution I stumbled upon the answer from “sawdust” in the following stackoverflow thread: https://stackoverflow.com/questions/37407667/ubi-error-ubi-io-read-error-74-ecc-error
Using “nand write.trimffs” instead of “nand write” seems to have solved the issue and I can reboot multiple times.
Best regards,
Thorsten