Kernel boot fails due to missing environment variable in Contao deployment

When deploying a Contao CMS project, the Symfony kernel fails to start and the application returns a 500 error. The goal is to determine which missing or incorrect environment variable in the .env (or .env.local) file is preventing the kernel from booting, based solely on the information available in the var/log directory.

Constraints include relying on log entries that may not explicitly name the variable, and the possibility that multiple variables affect the boot process. There is also uncertainty about whether the issue stems from a missing variable, an incorrect value, or a file permission problem that masks the underlying cause.

What specific environment variable must be defined for the Symfony kernel to initialize in Contao 4.13?
How can we isolate the responsible variable from log output without altering the deployment files?
Is there a reliable way to confirm that correcting this variable resolves the kernel boot failure?