Troubleshoot OWA / ASP.NET

Community,

I am trying to troubleshoot an issue after installing CU14.  OWA gives status 500 / "Something went wrong ;("

IIS (W3SVC1) Logs redirect saying code 500.  Authentication seems to be working, and ECP works fine.

I get an ASP.NET error for every OWA attempt:
Windows Event ID: 1309 / Source ASP.NET 4.0.30319.0

Event code: 3005 
Event message: An unhandled exception has occurred. 
Event time: 2/21/2024 7:11:56 AM 
Event time (UTC): 2/21/2024 12:11:56 PM 
Event ID: 656744f519224d20afbbd3beffbfcfc2 
Event sequence: 2 
Event occurrence: 1 
Event detail code: 0 
 
Application information: 
    Application domain: /LM/W3SVC/2/ROOT/owa-2690-133529911049605798 
    Trust level: Full 
    Application Virtual Path: /owa 
    Application Path: D:\Exchange Server\V15\ClientAccess\owa\ 
    Machine name: TEST-CARM-EXCH 
 
Process information: 
    Process ID: 16956 
    Process name: w3wp.exe 
    Account name: NT AUTHORITY\SYSTEM 
 
Exception information: 
    Exception type: TargetInvocationException 
    Exception message: Exception has been thrown by the target of an invocation.
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Owin.Loader.DefaultLoader.<>c__DisplayClass12.<MakeDelegate>b__b(IAppBuilder builder)
   at Owin.Loader.DefaultLoader.<>c__DisplayClass1.<LoadImplementation>b__0(IAppBuilder builder)
   at Microsoft.Owin.Host.SystemWeb.OwinAppContext.Initialize(Action`1 startup)
   at Microsoft.Owin.Host.SystemWeb.OwinBuilder.Build(Action`1 startup)
   at Microsoft.Owin.Host.SystemWeb.OwinHttpModule.InitializeBlueprint()
   at System.Threading.LazyInitializer.EnsureInitializedCore[T](T& target, Boolean& initialized, Object& syncLock, Func`1 valueFactory)
   at Microsoft.Owin.Host.SystemWeb.OwinHttpModule.Init(HttpApplication context)
   at System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers)
   at System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context)
   at System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context)
   at System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext)

Could not load file or assembly 'Newtonsoft.Json, Version=6.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' or one of its dependencies. The system cannot find the file specified.
   at Microsoft.AspNet.SignalR.DefaultDependencyResolver.RegisterDefaultServices()
   at Microsoft.AspNet.SignalR.DefaultDependencyResolver..ctor()
   at Microsoft.AspNet.SignalR.GlobalHost.<>c.<.cctor>b__13_0()
   at System.Lazy`1.CreateValue()
   at System.Lazy`1.LazyInitValue()
   at Microsoft.AspNet.SignalR.GlobalHost.get_DependencyResolver()
   at Owin.OwinExtensions.UseSignalRMiddleware[T](IAppBuilder builder, Object[] args)
   at Owin.MapExtensions.Map(IAppBuilder app, PathString pathMatch, Action`1 configuration)
   at Owin.MapExtensions.Map(IAppBuilder app, String pathMatch, Action`1 configuration)
   at Microsoft.Exchange.Clients.Owa2.Server.Core.notifications.SignalR.SignalRStartup.Configuration(IAppBuilder app)

 
 
Request information: 
    Request URL: https://localhost:444/owa/proxylogon.owa 
    Request path: /owa/proxylogon.owa 
    User host address: 127.0.0.1 
    User:  
    Is authenticated: False 
    Authentication Type:  
    Thread account name: NT AUTHORITY\SYSTEM 
 
Thread information: 
    Thread ID: 9 
    Thread account name: NT AUTHORITY\SYSTEM 
    Is impersonating: False 
    Stack trace:    at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Owin.Loader.DefaultLoader.<>c__DisplayClass12.<MakeDelegate>b__b(IAppBuilder builder)
   at Owin.Loader.DefaultLoader.<>c__DisplayClass1.<LoadImplementation>b__0(IAppBuilder builder)
   at Microsoft.Owin.Host.SystemWeb.OwinAppContext.Initialize(Action`1 startup)
   at Microsoft.Owin.Host.SystemWeb.OwinBuilder.Build(Action`1 startup)
   at Microsoft.Owin.Host.SystemWeb.OwinHttpModule.InitializeBlueprint()
   at System.Threading.LazyInitializer.EnsureInitializedCore[T](T& target, Boolean& initialized, Object& syncLock, Func`1 valueFactory)
   at Microsoft.Owin.Host.SystemWeb.OwinHttpModule.Init(HttpApplication context)
   at System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers)
   at System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context)
   at System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context)
   at System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext)

The health check script doesn't indicate .net issues, but I feel like this is either application pool related, or missing installdir\client access\owa\ files.

The prem folder that has all the versions doesn't have anything under 15.2.1544.4 accept an 'ext' folder.

This issue was replicated on two internal servers that were otherwise working fine before upgrade.

Any ideas what logs to go to next?  A way to enable OWA/ASP trace logging?