How to reproduce channel is busy throwing EROChannelBusy

My client access in C# the RO services as follows and is getting the error “channel is busy”. He is sending multiple requests at the same time through the following code:

var client = new HttpClient()
….
response = await client.SendAsync(request);

The request is going to http://servername:1234/XmlRpc
The body content is a valid XML-RPC request

I am not a C# expert. Could you please verify if the error “channel is busy” is not the problem of the RO servers, but the problem of the C# code above?

Could you advice a solution in C# how the RO services could be accessed?

Thank you very much for your help!