Technical Note: Web Server Error 404 when accessing GUI
Description
This article desctibes that the 'Web Server Error 404' is commonly seen when a web page or resource is not found on the server. If this error is encountered when trying to access the Graphical User Interface (GUI) of a device or software, it indicates that the requested GUI page is unavailable.
Scope
FortiGate.

Solution
Clear the cache of the web browser and then access the GUI again.
Conclusion:
The 'Web Server Error 404' when accessing the GUI is generally a straightforward issue to resolve. Clearing the browser cache is often the simplest solution, but if the problem persists, further diagnostics might be required.