Bad Request & Internal Server error
When the server of URL cannot understand the request it has received it responds with a Bad request or a malformed syntax error message. If there is an error in processing the request received by the server, it responds with an Internal Server error message. Common reasons for this are, incorrectly formatted POST data, incorrect URL format and unexpected User Agent.
Incorrectly formatted POST data
If the URL is HTTP method "POST", then ensure that the POST data type configured is correct. You can verify this by cross checking any documentation of the URL or by making a request with POST data to the URL using cURL/Wget, if the response code is 400 or 500, then the POST data sent to the URL is of the wrong format
For Internal Server error, check the URL's server logs to see what exception is occurring while processing the request. At times, bugs in the server may lead to error while processing requests.
Incorrect URL format
URLs added in Applications Manager will be URL encoded when a request is made to them. Hence if you have added an already encoded URL for monitoring, then there will be double encoding while making the request. This can cause an issue in connecting to the URL successfully. Hence, you don't need to encode a URL while creating a monitor for it.
Unexpected User Agent
Certain servers are designed to reject requests originating from sources other than browsers. In such cases, edit the User Agent of the URL monitor. You can get your User-Agent's value from https://www.whatismybrowser.com/detect/what-is-my-user-agent.
After entering the value in the edit monitor screen, update the URL monitor and poll the monitor a couple of times and check.