Exchange 2019 - OWA 500 Error after login

Hey out there,

we have a problem with an Exchange 2019 Server. OWA throws a 500 error after login with no information under details. This happens with internal and external access (Server behind a reverse proxy)

Exchange is an 2019 CU14 with latest SU installed on a Windows Server 2022 Standard VM.

EAC is working fine.

What I've done so far:

• Restart Server ;-)
• Recreated Virtual Directory for OWA
• Reinstalled latest SU
• Create new Auth Certificate
• Update CAS and Assemblies
• Checked the bindings in the IIS

After trying to login it shows me the following error in the eventlog of the Server:

Event code: 3005 

Event message: Es ist eine unbehandelte Ausnahme aufgetreten. 

Event time: 02.08.2024 08:18:50 

Event time (UTC): 02.08.2024 06:18:50 

Event ID: 882fcea0bf0945ec9dc358d4846b40c5 

Event sequence: 2 

Event occurrence: 1 

Event detail code: 0 

Application information: 

    Application domain: /LM/W3SVC/2/ROOT/owa-633-133670531266125478 

    Trust level: Full 

    Application Virtual Path: /owa 

    Application Path: E:\ExchangeServer\ClientAccess\owa\ 

    Machine name: <EX-Server-Name> 

Process information: 

    Process ID: 10796 

    Process name: w3wp.exe 

    Account name: NT-AUTORITÄT\SYSTEM 

Exception information: 

    Exception type: HttpException 

    Exception message: Die Datei oder Assembly "Microsoft.Exchange.Services.Json, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" oder eine Abhängigkeit davon wurde nicht gefunden. Das System kann die angegebene Datei nicht finden.

   bei System.Web.HttpApplicationFactory.EnsureAppStartCalledForIntegratedMode(HttpContext context, HttpApplication app)

   bei System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers)

   bei System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context)

   bei System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context)

   bei System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext)

Die Datei oder Assembly "Microsoft.Exchange.Services.Json, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" oder eine Abhängigkeit davon wurde nicht gefunden. Das System kann die angegebene Datei nicht finden.

   bei Microsoft.Exchange.Clients.Owa2.Server.Core.BaseApplication.ExecuteApplicationStart(Object sender, EventArgs e)

   bei Microsoft.Exchange.Clients.Owa2.Server.Core.Global.Application_Start(Object sender, EventArgs e)

Request information: 

    Request URL: https://localhost:444/owa/exhealth.check 

    Request path: /owa/exhealth.check 

    User host address: ::1 

    User:  

    Is authenticated: False 

    Authentication Type:  

    Thread account name: NT-AUTORITÄT\SYSTEM 

Thread information: 

    Thread ID: 43 

    Thread account name: NT-AUTORITÄT\SYSTEM 

    Is impersonating: False 

    Stack trace:    bei System.Web.HttpApplicationFactory.EnsureAppStartCalledForIntegratedMode(HttpContext context, HttpApplication app)

   bei System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers)

   bei System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context)

   bei System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context)

   bei System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext)

Custom event details: