I got Twitter to work after some effort - you have to make sure your X developer app is inside a "project" so that it has access to the v2 API as well.
...I'm now having problems with LinkedIn that I can't seem to debug at all.
Thx @nebulon ! This could be part of the First Time Usage hints or docs at https://docs.cloudron.io/apps/postiz/ - just not a very obvious behaviour by Postiz