I know this topic is old, but I came across it in my search.
I had all kinds of problems just trying a straight upgrade (from 2.27). I tried everything: web UI, u-boot web UI at 192.168.1.1, even tried sysupgrade command, none of it worked for me.
Some people had success going from 3.105 to 3.216 to 4.3.17 (as mentioned here). I did not try that way.
What worked for me was flashing vanilla OpenWrt, and then 4.3.18. I can't find link to the post where I got that idea (and linked to correct OpenWrt version), but I will edit this if I come across it again.