Hi at all,
After several failed upgrades of Ubuntu 14 to 16 yesterday I got a real good Ubuntu upgrade support thread by @UFHH01.
Before I alwaya did the upgrade only by do-release-upgrade.
But I have a default installation by 1and1 Plesk 12.5 on Ubuntu 14.04.5 LTS.
So I activated Plesk 12.5...