Allright, thanks for clearing that up. You might want to include this info in your documentation. ;)
I'll try the upgrade to 16.04 again soon and post back in a new thread if i run into anything odd.
I'd like to point out that https://kb.plesk.com/en/126808 specificly says "Applies to: Plesk 12.5 for Linux". It also explicitly states that Plesk 12.5 does not support Ubuntu 16.04.
There is however an other article here...