If there is a content regexp mismatch, the alert says “Address not available” (and the email says “Connection failed” which isn’t true because the connection did not fail). Reproducible with v 5.27.1.
check host example.com with address example.com if failed port 80 protocol http and request "/" with content != '<!doctype html>.*' with timeout 10 seconds then alert
This will result in these verbose log messages
Socket test failed for [18.104.22.168]:80 -- HTTP error: Regular expression matches Socket test failed for [2606:2800:220:1:248:1893:25c8:1946]:80 -- Address not available 'example.com' failed protocol test [HTTP] at [example.com]:80/ [TCP/IP] -- Address not available Sending Connection failed notification to firstname.lastname@example.org
Changing the comparison to
= will result in no alert.