I have Umbraco version 7.4.3 and I deployed it on staging server successfully and it was working until recently it stopped working for backoffice only. As for frontend it works just fine.

Also it still work on Safari and IE / Edge but not on Chrome and Firefox.

Here is error that I can see on chrome Console.

Failed to load resource: the server responded with a status of 404 (Not Found)
http://staging.justt.com/umbraco/ServerVariables?umb__rnd=fffffffc656dd456&umb__rnd=7.4.3.826540469 Failed to load resource: the server responded with a status of 401 (You must login to view this resource.)
DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:11 No url found for api name legacyTreeJs(anonymous function) @ DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:11
http://staging.justt.com/login.aspx?ReturnUrl=%2fumbraco%2fbackoffice%2fUmbracoApi%2fAuthentication%2fGetRemainingTimeoutSeconds Failed to load resource: the server responded with a status of 404 (Not Found)
DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:11 TypeError: Cannot read property 'keepUserLoggedIn' of undefined
    at DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:1175
    at DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:15
    at e (DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:7)
    at DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:8(anonymous function) @ DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:11(anonymous function) @ DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:10(anonymous function) @ DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:15e @ DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:7(anonymous function) @ DependencyHandler.axd?s=L3VtYnJhY28vbGliL2pxdWVyeS9qcXVlcnkubWluLmpzOy91bWJyYWNvL2xpYi9hbmd1bGFyLzE…:8