Using an apim in swedencentral to connect to azure AI Services with semantic kernel, a assistants group chat runs into an error

Hi!
We have an .net application using the latest Semantic-Kernel package to build up a assistants group chat. We connect through an APIM to the AI services (Model Deployment gpt-4o). All needed apis and operations are in place. Since mid of April, we run into an error in sedencentral when calling InvokeStreamingAsync() on the group chat. Using InvokeAsync() works fine! We also don't run into errors when we use a different region (we tried francecentral, east us 2, ....).
My demo solution is based on this one: https://learn.microsoft.com/en-us/semantic-kernel/frameworks/agent/examples/example-agent-collaboration?pivots=programming-language-csharp
But we use OpenAIAssistantAgent instead:

AssistantClient assistantClient = OpenAIAssistantAgent.CreateAzureOpenAIClient(
    new ApiKeyCredential(apiKey),
    new Uri(endpoint),
    httpClient: httpClient).GetAssistantClient();

Assistant agentReviewer = await assistantClient.CreateAssistantAsync(
    deploymentName, new AssistantCreationOptions()
    {
        Name = ReviewerName,
        Instructions =
            """
            ...
            """,
        Description = "Your responsibility is to review and identify how to improve user provided content.",
        
    });

OpenAIAssistantAgent assistantAgentReviewer = new OpenAIAssistantAgent(
    agentReviewer,
    assistantClient);

[...]


chat.AddChatMessage(new ChatMessageContent(AuthorRole.User, input));

chat.IsComplete = false;

await foreach (var response in chat.InvokeStreamingAsync())

Application insights shows perfectly that it stops with the threads/.../runs call but without an error:

See here the call stack of the error:

System.NullReferenceException: Object reference not set to an instance of an object.
   at OpenAI.AsyncSseUpdateCollection`1.AsyncSseUpdateEnumerator`1.System.Collections.Generic.IAsyncEnumerator<U>.MoveNextAsync()
   at OpenAI.AsyncSseUpdateCollection`1.GetValuesFromPageAsync(ClientResult page)+MoveNext()
   at OpenAI.AsyncSseUpdateCollection`1.GetValuesFromPageAsync(ClientResult page)+MoveNext()
   at OpenAI.AsyncSseUpdateCollection`1.GetValuesFromPageAsync(ClientResult page)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()
   at System.ClientModel.AsyncCollectionResult`1.GetAsyncEnumerator(CancellationToken cancellationToken)+MoveNext()
   at System.ClientModel.AsyncCollectionResult`1.GetAsyncEnumerator(CancellationToken cancellationToken)+MoveNext()
   at System.ClientModel.AsyncCollectionResult`1.GetAsyncEnumerator(CancellationToken cancellationToken)+MoveNext()
   at System.ClientModel.AsyncCollectionResult`1.GetAsyncEnumerator(CancellationToken cancellationToken)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()
   at Microsoft.SemanticKernel.Agents.OpenAI.Internal.AssistantThreadActions.InvokeStreamingAsync(OpenAIAssistantAgent agent, AssistantClient client, String threadId, IList`1 messages, RunCreationOptions invocationOptions, ILogger logger, Kernel kernel, KernelArguments arguments, CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Agents.OpenAI.Internal.AssistantThreadActions.InvokeStreamingAsync(OpenAIAssistantAgent agent, AssistantClient client, String threadId, IList`1 messages, RunCreationOptions invocationOptions, ILogger logger, Kernel kernel, KernelArguments arguments, CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Agents.OpenAI.Internal.AssistantThreadActions.InvokeStreamingAsync(OpenAIAssistantAgent agent, AssistantClient client, String threadId, IList`1 messages, RunCreationOptions invocationOptions, ILogger logger, Kernel kernel, KernelArguments arguments, CancellationToken cancellationToken)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()
   at Microsoft.SemanticKernel.Diagnostics.ActivityExtensions.RunWithActivityAsync[TResult](Func`1 getActivity, Func`1 operation, CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Diagnostics.ActivityExtensions.RunWithActivityAsync[TResult](Func`1 getActivity, Func`1 operation, CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Diagnostics.ActivityExtensions.RunWithActivityAsync[TResult](Func`1 getActivity, Func`1 operation, CancellationToken cancellationToken)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()
   at Microsoft.SemanticKernel.Agents.AgentChat.InvokeStreamingAgentAsync(Agent agent, CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Agents.AgentChat.InvokeStreamingAgentAsync(Agent agent, CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Agents.AgentChat.InvokeStreamingAgentAsync(Agent agent, CancellationToken cancellationToken)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()
   at Microsoft.SemanticKernel.Agents.AgentGroupChat.InvokeStreamingAsync(Agent agent, CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Agents.AgentGroupChat.InvokeStreamingAsync(Agent agent, CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Agents.AgentGroupChat.InvokeStreamingAsync(Agent agent, CancellationToken cancellationToken)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()
   at Microsoft.SemanticKernel.Agents.AgentGroupChat.InvokeStreamingAsync(CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Agents.AgentGroupChat.InvokeStreamingAsync(CancellationToken cancellationToken)+MoveNext()
   at Microsoft.SemanticKernel.Agents.AgentGroupChat.InvokeStreamingAsync(CancellationToken cancellationToken)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()

We tried to update semantic kernel with no success.
When we connect directly to the AI Services instead using the apim, everything works fine!
So, it looks like something is wrong with the apim service in swedencentral!
We have the whole infrastructure defined in bicep, so we are sure that the exact same setup works in other regions.

I would be glad if you can help, finding the error.