You have an exceptq report too. I suspect it's similar to the one
generated here.
It turns out that I encountered the same upgrade failure back in March
which is when I upgraded these databases. I probably retried the upgrade
and did not think too much of it when the retry worked. This explains the
lack of useful notes. However, my exceptq report archives include the
report for this mysql failure.
I also ran into the now expected innodb issues. This set of databases ran
with skip-innodb and 5.6 no longer allows this.