Meaning of the trouble message - "Content Length Modified"
Under Threshold Configuration, if you have the option Notify when website content is modified selected, then any minor textual change on your website will reflect as a trouble alert.
In the Site24x7 portal, log in and navigate to Admin > Configuration Profiles > Threshold and availability > Add Threshold Profile > Threshold Configuration > Notify when website content is modified
- Notify when website content is modified: Select the check box to get notified when there is a change or modification in your website content.
- Notify when website content changes in percentage: Enter a value in percentage to get notified when the content in your website is changed beyond the specified value.

You can specify a threshold percentage for content changes, which will trigger a notification if exceeded, or you can specify keywords to be present under the 'Content Match' tab.