This KB describes a scenario in which the browser console is generating the following error: "Failed to load resource: the server responded with a status of 404 (Not Found)" for a UI Script. In particular, the error remains even after the UI Script mentioned has been de-activated or even deleted from the instance.
The cause can be found in Service Portal > Dependencies. Widget Dependency [sp_dependency] records have a Related List of JS Includes. The portal(s) configured on this dependency record will load the UI Script even if it's set to false.
Refactor any JS Includes list to no longer include the script. You may also find just unchecking 'Include on page load' field on the [sp_dependency] record will be enough to resolve the error.
You can easily find if the UI Script in question is impacted by this JS Includes behavior by navigating directly to "m2m_sp_dependency_js_include.list" and searching the UI Script column for the name of the script.