I have upgraded a site from EPiServer 7.5 to 11.8.
When deploying the upgraded site to the test environment the edit and admin interface stopped working (the top episerver ui menu visible but lots of js errors and no “main edit window visible” and entire admin mode 404 not found).
The problem was related to some old config for “ProtectedAddons”, i just commented the below config in web.config on test server and the edit and admin mode works again.
(comment out/remove below from web.config)
<add name="ProtectedAddons" virtualPath="~/epi/" physicalPath="[appDataPath]\Modules" type="EPiServer.Web.Hosting.VirtualPathNonUnifiedProvider, EPiServer.Framework.AspNet" />
Inspiration for the solution came from here: Object reference error in initialization modules