Also the process log (-log /path/to/caddy.log) given as a flag. Alternately to stdout if you’ve got something else logging the program’s output. You haven’t actually specified how you’re actually running Caddy in this environment, so I’m not sure what’s appropriate.
Anyway, If Caddy’s breaking somehow, that’s likely where we’ll see an error or panic, and it’s also where we’ll see non-site-specific errors, since errors and log are both specific to the site you configure them for.