Why is Rebus.Send not working after update to .NET 5 using Azure Service Bus

We have a .net core 3.1 web project running with Rebus and Azure Service Bus.

After updating to .NET 5 we can no longer send messages to the Azure Service Bus queue using Rebus. No code changes have been made, only update to .NET 5 and latest Rebus-packages(6.6.1).

var contractMessage = _manager.GetContractMessage();
await _bus.Send(contractMessage);

This throws:

Exception
    Rebus.Exceptions.RebusApplicationException: Could not send to queue 'sbq-queue-queue1'
 ---> System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at Microsoft.Azure.ServiceBus.Core.MessageSender.OnSendAsync(IList`1 messageList)
   at Microsoft.Azure.ServiceBus.RetryPolicy.RunOperation(Func`1 operation, TimeSpan operationTimeout)
   at Microsoft.Azure.ServiceBus.RetryPolicy.RunOperation(Func`1 operation, TimeSpan operationTimeout)
   at Microsoft.Azure.ServiceBus.Core.MessageSender.SendAsync(IList`1 messageList)
   at Rebus.AzureServiceBus.AzureServiceBusTransport.<>c__DisplayClass36_0.<<GetOutgoingMessages>g__SendOutgoingMessagesToDestination|3>d.MoveNext()
   --- End of inner exception stack trace ---
   at Rebus.AzureServiceBus.AzureServiceBusTransport.<>c__DisplayClass36_0.<<GetOutgoingMessages>g__SendOutgoingMessagesToDestination|3>d.MoveNext()
--- End of stack trace from previous location ---
   at Rebus.AzureServiceBus.AzureServiceBusTransport.<>c__DisplayClass36_1.<<GetOutgoingMessages>g__SendOutgoingMessages|1>d.MoveNext()
--- End of stack trace from previous location ---
   at Rebus.Transport.TransactionContext.InvokeAsync(Func`2 actions)
   at Rebus.Transport.TransactionContext.Complete()
   at Rebus.Bus.RebusBus.InnerSend(IEnumerable`1 destinationAddresses, Message logicalMessage)
   at Rebus.Bus.RebusBus.Send(Object commandMessage, IDictionary`2 optionalHeaders)
   at xx.xx.xx.xx.xx.someclass.request.mediatrHandler.Handle(ClassRequest request, CancellationToken cancellationToken) in /home/vsts/work/1/s/xx.xx.xx/Application/xx/xx/xxRequest.cs:line 187

The weird thing is this seems to work when debugging locally, but not on production servers, which are azure virtual machines running IIS. Same hosting bundle as when testing locally in IIS.

To connect to the ASB I have tried both managed identity and full connection string.

Using packages:

Rebus 6.6.1
Rebus.AzureServiceBus 9.0.5
Rebus.Serilog 6.0.0
Rebus.Serviceprovider 6.4.1

And setting up Rebus in the ConfigureServices:

  services.AddRebus((options, serviceProvider) => options
    .Logging(l => l.Serilog())
    .Transport(t => t.UseAzureServiceBusAsOneWayClient(configuration["RebusConfig:AzureServiceBusConnectionString"]))
    .Routing(r => {
      r.TypeBased().MapAssemblyOf<Contract1>(configuration["RebusConfig:Routes:Route1"])
        .MapAssemblyOf<Contract2>(configuration["RebusConfig:Routes:Route2"]);
    })
  );

After updating Microsoft Azure packages to the latest version and switching the connections to the new DefaultAzureCredentials() for all azure services this resolved itself.