With #Zabbix 6.0 one of our web checks started failing with `regexp_exec() failed with error -8` when looking for a required string. This basically means “maximum recursion exceeded” (so something went into a loop). Turns out the site in question had a duplicated `</head>` tag which makes the HTML invalid.

So apparently something changed with the RegEx check and valid HTML is now required as well 🧐