Thanks a lot for the advice. Had exactly the same problem with our Bitnami Mattermost Team Edition Installation on AWS after upgrading to 4.3.1
Problem here was, that Bitnami’s upgrade Maunal (can be found here: https://docs.bitnami.com/aws/apps/mattermost/#how-to-upgrade-mattermost) includes this command after the upgrade is almost complete:
sudo chown -R root:root /opt/bitnami/apps/mattermost/server
After this, mattermost (which is also the system user who runs mattermost) could not change the config file.
With “sudo chown -c mattermost:mattermost /opt/bitnami/apps/mattermost/server/config/config.json” I could fix this issue.