Existing HTTP monitor returns Error 9501 "Not Matched" for a URL which is subsequently redirected.
APP SYNTHETIC MONITOR
The Match string test only runs against the original/monitored page and does not run on the final "redirected to" page.
In this case only the "redirected to" page had the string present.
As this monitor was working before, something must have changed with the monitor definition (match string added) or monitored page content changed.
If the string trying to be matched does not exist in the monitored URL and only exists on "redirected to" URL, then remove the "Match string" condition.
Another similar Error: (9599) Not matched (no content), which happens with the Monitor configure with "Match string or regular expression", The "Match string or regular expression" "<value>" is not being found on the page for some reason.
It also can be caused the match string not on the monitor page, or the part of page loading up after timeout.