If you use the same solution than @TheStealth, you need to edit the app.yml file and rebuild
cd /var/discourse
nano containers/app.yml
Around line 38, change this:
## Which Git revision should this container use? (default: tests-passed)
#version: tests-passed
to
## Which Git revision should this container use? (default: tests-passed)
version: stable
Save the file, then:
./launcher rebuild app
Given the error message, I would assume it’s related to this page:
/admin/config/flags
I would delete the custom flags you might have created in the past. Then try to rebuild on the tests-passed branch again