Discourse 2.6 branch was almost 2 years ago, so there is a possibility that it just doesn’t work with the current version of the base container image we ship currently. In order to troubleshoot, I would need the ./launcher rebuild logs of trying to rebuild it as 2.6.7 in a brand new install.
However, running a Discourse version that doesn’t receive security fixes anymore is a really bad idea, so the correct way forward is fixing the software you use to handle this integration.
Maybe you can share the SQL query that needs fixing?