This screen failed on me after updating to EPiServer 11.
[NullReferenceException: Object reference not set to an instance of an object.]
EPiServer.UI.Admin.SiteInformationEdit.OnLoad(EventArgs e) +57
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1376
By using dotPeek I found that this was caused by the EPiServer.UI.Admin assembly which referneced a control "SiteIdPanel" which didn't exist in the /modules/_protected/CMS/.../SiteInformationEdit.aspx file. The faulty CMS.zip this file resided in was version 126.96.36.199. I had to update the Episerver Vs2015 extensions, create an Alloy example site and copy the provided 11.5.0 CMS.zip to my project to make the ManageWwebsites screen work again.
Can someone explain why this file isn't updated automatically when I update EPiServer packages via Nuget?
It should. However it might have failed because of "reasons", for example that CMS.zip was being used by other process
run upgrades in VS with site "shutdown" (kill the process it it's running) - because depending on your site characteristics you might not visiting the site, but iis worker process is alive and might keep files locked. and i've seen that nuget sometimes shallows excpetions when not able to update some of the files.