Monitor reports DOWN although the endpoint returns 200

That is exactly it. The comparison is a plain case-sensitive substring match, not JSON parsing, so a single space breaks it. To fix: 1. Open the monitor and edit the expected body text so it matches the raw response byte for byte. For compact JSON that means no space after the colon. 2. Safer still, use a short distinctive fragment rather than a full key-value pair. Something like ok or healthy is much less brittle. 3. If you do not need body validation at all, clear the field. With it empty, the check passes on any 2xx response. 4. Save and wait one check interval, or shorten the interval temporarily to see the result faster. While you are in there, one related setting worth knowing: if you leave expected status codes empty, any 2xx counts as up. If you fill it in, only the codes you list count. Those two validations run in order, status first, then body, then headers, and the first failure is what you see in the error message.