Unexpected 40x or 50x Error from ASM HTTP/S Monitor

Document ID : KB000105944
Last Modified Date : 13/07/2018
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While monitoring a website using an ASM HTTP/S monitor, an unexpected 40x or 50x error is returned in ASM. This issue occurs even though the url is working and fully accessible in a browser.
ASM version 10.x
One common cause of this issue is that a missing request header in the request from ASM is causing an unexpected response from the Website. Browsers typically add these requests headers automatically preventing the issue from occurring for users accessing the site in a browser. 
To correct the issue, try adding each request header sent in your browser to the "Custom HTTP request headers" section of the Advanced tab of your monitor configuration. The request headers sent can be obtained by reviewing the request using the developer tools of your browser. Copy each header into the "Custom HTTP request headers" field and retest the monitor. Should errors persist, please contact CA Support.