upgrade from 150007 to 160000 FAILED: could not execute: command terminated with exit code 1

[docbobo]

I have two databases that were started with version 15 that I'd like to upgrade to 16. Unfortunately, that does not work and the operator reports

major version upgrade failed: could not execute: command terminated with exit code 1

That said, I did not find anything obvious in the logs that is hinting at the actual reason.

Any suggestions?