Thanks for your reply.
So, is the documentation wrong on that part? As it addresses package installations.
Quote from documentation: (link in my first post)
If you installed the Zammad DEB or RPM package
$ zammad run rake db:drop
$ zammad run rake db:create
$ zammad run rake db:migrate
$ zammad run rake db:seed
What I did now:
I changed to user postgres and created the database manually with
psql -c "CREATE DATABASE zammad OWNER zammad;"
After that the migrate and seed commands ran without errors.
Is that a proper way to do it? Or could it lead to problems when updating the package later?
To sum it up (for me): User zammad may not create the database, but user postgres may not execute the zammad run rake ... commands, only zammad and root may. Therefore I cannot just send the four commands as postgres.
Is that the problem behind the error I got?