do-release-upgrade -d今天在 16.04 机器上尝试了一下，但
Upgrades to the development release are only
available from the latest supported release.
LTS 到 LTS 肯定是允许的吗？
结果cat /etc/update-manager/release-upgrades是Prompt=lts
答案1
删除所有文件~/.cache/update-manager-core/并重试升级过程。