Starting from Confluence 9.1, when migrating using XML backups, you need to copy the secrets-config.yaml, keys and secured to the new shared home directory before installing Confluence. It has been updated in https://confluence.atlassian.com/doc/migrating-to-another-database-148867.html (Step 2.3 and 6).

If a secret was not copied to the new shared home, Confluence will fail to start but doesn't clearly indicate what caused the failure - not even in logs. It would be helpful for administrator if the reason for Confluence to fail to start to be indicated on the UI.

Also, it would be nice to have a message on the UI when generating Site XML backup that secrets-config.yaml, keys/ and secured/ needs to be manually copied over to the destination server when restoring the Sit XML backup.