The problem lies likely in your old build config, if those are your own snapshots and you can't sysupgrade from them.
mtd is still there? usable from command line?
The symptom is not typical, so I wonder if you have somehow lost the flashing ability.
You have apprently modified the build somewhat? ipv6 left out?
That change should not affect the sysupgrade, but have you left out any core packages?