Acumatica - Error when upgrading to 2020R2 - Workspace menu items disappear - acumatica

Question

We are encountering an error when upgraded to 2020R2, specifically from 2020R1 (Build 20.109.0032) to 2020R2 (Build 20.204.0032).
Upon successful login, the user is taken to the Default page (ScreenID = 00000000) and receives the error "Object reference not set to an instance of an object." popup message. When dismissed, the page renders its Welcome information, but the left-hand Workspaces and menu items (Finance, Sales Orders, Receivables, etc.) have disappeared, instead showing a spinning icon. Quick Search at page top offers no menu items when searched.
The user can manually enter any page (eg. ScreenID=SO301000) directly into the Address URL and reach the page content, but again initially gets the error and has no workspace menus.
There is nothing in the Trace to give clues. However, the browser's Web Console give details as shown below:
XHRGEThttp://localhost/AcumaticaERP/(W(1))/frameset/sitemap
[HTTP/1.1 500 Internal Server Error 912ms]
Message "An error has occurred."
ExceptionMessage "Object reference not set to an instance of an object."
ExceptionType "System.NullReferenceException"
StackTrace " at PX.Web.UI.Frameset.Services.AccessProvider.ApplyRestrictionsForCurrentUser(Screen screen)\r\n at PX.Web.UI.Frameset.Services.SiteMapRepository.<>c__DisplayClass21_0.<GetSiteMapImpl>b__10(Screen s)\r\n at AutoMapper.Mapper.AutoMapper.IMapper.Map(Object source, Type sourceType, Type destinationType, Action`1 opts)\r\n at AutoMapper.Mapper.AutoMapper.IMapper.Map[TDestination](Object source, Action`1 opts)\r\n at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()\r\n at System.Li…HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__3.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__15.MoveNext()"
If I place a breakpoint in VS2019 at the beginning of Page_Init() of PageTitle.ascx.cs, the error occurs first, then the breakpoint is reached:
protected void Page_Init(object sender, EventArgs e)
{
var lifetimeScope = this.Context.GetLifetimeScope(); <-- set a breakpoint here
if (lifetimeScope != null)
{
screenRepository = lifetimeScope.Resolve<IScreenRepository>();
}
else
....
A similar upgrade of a demo website and database with Sales Demo data works just fine.
We conducted a SQL Schema Compare of the two databases and opted to Update the differences, which were all examples of moving away from the NoteID column allowing NULL in many tables.
The schemas are now the same, the website upgrades are the same, so now I suspect it is the data... somewhere... causing a lack of workspace and menus.
The call to PX.Web.UI.Frameset.Services.AccessProvider.ApplyRestrictionsForCurrentUser() seems to be the issue but is data-related in my case. It's as if AccessProvider of screenRepository isn't available yet in a different control upstream from use of PageTitle.ascx?
Where can I look next to try to resolve?
Thank you.