I entered a longer String to be checked and I received error mails over hours with " webtest failed: Page didn't match" always at the same step. (I worte a script that reads out the error messages and sends it via email)
I am definitely sure that the page AND the String are available.
I think that zabbix checks the availability of the website and splits this verification up into about 4 KB parts and that the String that shall be checked also gets split up.