Hmmm, that was quite simple:
- ./launcher enter web_only
- sudo apt-get install postgresql
This upgrades the postgres package to V. 18.
And then start the backup from Admin UI which seems to be successful
[2026-05-21 16:26:50] 'admin' has started the backup!
[2026-05-21 16:26:50] Marking backup as running...
[2026-05-21 16:26:50] Making sure '/var/www/discourse/tmp/backups/default/2026-05-21-162650' exists...
[2026-05-21 16:26:50] Making sure '/var/www/discourse/public/backups/default' exists...
[2026-05-21 16:26:50] Updating metadata...
[2026-05-21 16:26:50] Dumping the public schema of the database...
[2026-05-21 16:26:50] pg_dump: executing SELECT pg_catalog.set_config('search_path', '', false);
[2026-05-21 16:26:50] pg_dump: last built-in OID is 16383
[2026-05-21 16:26:50] pg_dump: reading extensions
[2026-05-21 16:26:50] pg_dump: identifying extension members
[2026-05-21 16:26:50] pg_dump: reading schemas
[2026-05-21 16:26:50] pg_dump: reading user-defined tables
[2026-05-21 16:26:50] pg_dump: reading user-defined functions
[2026-05-21 16:26:50] pg_dump: reading user-defined types
[2026-05-21 16:26:50] pg_dump: reading procedural languages
[2026-05-21 16:26:50] pg_dump: reading user-defined aggregate functions
[2026-05-21 16:26:50] pg_dump: reading user-defined operators
[2026-05-21 16:26:50] pg_dump: reading user-defined access methods
[2026-05-21 16:26:50] pg_dump: reading user-defined operator classes
[2026-05-21 16:26:50] pg_dump: reading user-defined operator families
[... .... ...]
[2026-05-21 16:26:57] Finalizing backup...
[2026-05-21 16:26:57] Creating archive: netzwissen-forum-2026-05-21-162650-v20260520064255.tar.gz
[2026-05-21 16:26:57] Making sure archive does not already exist...
[2026-05-21 16:26:57] Creating empty archive...
[2026-05-21 16:26:57] Archiving data dump...
[2026-05-21 16:26:57] Archiving uploads...
[2026-05-21 16:26:58] Removing tmp '/var/www/discourse/tmp/backups/default/2026-05-21-162650' directory...
[2026-05-21 16:26:58] Gzipping archive, this may take a while...