Can't login on Jellyfin v12 rc5

[avdept]

As reported by fellow redditor here https://www.reddit.com/r/jellyfin/comments/1vnhkco/comment/p3qbyaz/

[2026-08-14 23:47:47.985 +02:00] [ERR] [16] Jellyfin.Api.Middleware.ExceptionMiddleware: Error processing request. URL "POST" "/Users/AuthenticateByName".
System.ArgumentNullException: Value cannot be null. (Parameter 'request.App')
   at System.ArgumentNullException.Throw(String paramName)
   at System.ArgumentNullException.ThrowIfNull(Object argument, String paramName)
   at System.ArgumentException.ThrowNullOrEmptyException(String argument, String paramName)
   at System.ArgumentException.ThrowIfNullOrEmpty(String argument, String paramName)
   at Emby.Server.Implementations.Session.SessionManager.AuthenticateNewSessionInternal(AuthenticationRequest request, Boolean enforcePassword)
   at Jellyfin.Api.Controllers.UserController.AuthenticateUserByName(AuthenticateUserByName request)
   at lambda_method9649(Closure, Object)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(ActionContext actionContext, IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()