X_SECRETS_KEY_MISSING
Means
no master key for the encrypted secrets file
Typical cause
ULTIMATE_SECRETS_KEY is unset and .secrets.key does not exist, while a sealed file does. Fatal at boot on purpose — an app that started without its secrets authenticates against nothing and still reports healthy
Fix
export ULTIMATE_SECRETS_KEY="$(cat .secrets.key)", or x secrets init in a repo that has no key yet