I have just gone through the whole U-boot flashing fail error procedure, I am now back to the latest beta firmware.
My best smoothly flashing “guide” is as the following 
After try&error for couple times, the “best” flashing “path” is as the following:
v3.001(Uboot img) OR v3.104(Uboot img) → v.3.105 (sysupgrade tar through web gui local upgrade) -->v.3.212 (web gui system auto check upgrade) → latest 3.215 beta 2 (local upgrade)
What an exciting flashing journey 