OK, as always, it was a “layer 8 problem” 
I already had a discourse database in the postgres but the discourse user/role had not all required privileges on it. And I did miss some major “behavioural” differences between mariadb and postgres …
Now I re-created all: user, database, permissions for the user/role (including ALTER DEFAULT PRIVILEGES FOR ROLE discourse IN SCHEMA public GRANT ALL ON TABLES TO “discourse”; to match tables created by this user in the future)
The build finished and the instance runs fine with the remote postgres server, so its ready for importing the backup from the older instance.