Event ID 1309 ASP.NET 4.0.30319.0 RDWeb Server 2019

I'm having a problem with slow RDWeb access through HTML5. If I open more than 4-5 sessions in the same browser window, the CPU on the client machine accessing RDWeb starts spiking and the webpage crashes.

The only issue I can find is I'm getting this event viewer warning about every 15-20 minutes:

Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 3/11/2022 8:55:03 AM
Event time (UTC): 3/11/2022 2:55:03 PM
Event ID: 238c36ca09574771bba31bc342396708
Event sequence: 864
Event occurrence: 26
Event detail code: 0

Application information:
Application domain: /LM/W3SVC/1/ROOT/RDWeb/Pages-1-132914397185506303
Trust level: Full
Application Virtual Path: /RDWeb/Pages
Application Path: C:\Windows\Web\RDWeb\Pages\
Machine name: [REMOVED FOR PRIVACY]

Process information:
Process ID: 5668
Process name: w3wp.exe
Account name: IIS APPPOOL\RDWebAccess

Exception information:
Exception type: NullReferenceException
Exception message: Object reference not set to an instance of an object.
at Microsoft.TerminalServices.Publishing.Portal.FormAuthentication.TSFormAuthTicketInfo..ctor(HttpContext objHttpContext)
at ASP.en_us_default_aspx.<GetAppsAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Web.UI.PageAsyncTaskManager.<ExecuteTasksAsync>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.UI.Page.<ProcessRequestAsync>d__554.MoveNext()

Request information:
Request URL: https://[REMOVED FOR PRIVACY]:443/RDWeb/Pages/en-US/Default.aspx?ReturnUrl=../WebFeed.aspx
Request path: /RDWeb/Pages/en-US/Default.aspx
User host address: [REMOVED FOR PRIVACY]
User:
Is authenticated: False
Authentication Type:
Thread account name: IIS APPPOOL\RDWebAccess

Thread information:
Thread ID: 40
Thread account name: IIS APPPOOL\RDWebAccess
Is impersonating: False
Stack trace: at Microsoft.TerminalServices.Publishing.Portal.FormAuthentication.TSFormAuthTicketInfo..ctor(HttpContext objHttpContext)
at ASP.en_us_default_aspx.<GetAppsAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Web.UI.PageAsyncTaskManager.<ExecuteTasksAsync>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.UI.Page.<ProcessRequestAsync>d__554.MoveNext()

Custom event details: