Comment 5 for bug 2032851

I'm not sure if G is saying something similar above, but I found that deleting all the subfolders in cache when still on Ubuntu 18.04 allows for do-release-upgrade to complete successfully:

sudo find /etc/apparmor.d/cache/* -type d -exec rm -r {} \;

There were other files in the cache folder, but I assume that rm can handle those. I wasn't sure if it was safe to delete the cache folder, but it didn't appear to be present after do-release-upgrade completed.