RestSharp Execute blocks on Linux Ubuntu (but runs fine on Mac Mojave)

I needed to change from async RestSharp call ExecuteAsync to a sync call Execute. It tested fine on my Mac but when placed to staging Linux Ubuntu environment the whole program comes to a halt and the Rest call never goes out to the target server.

I have it like this:

1. An endless thread taking the items out of queue.
2. For each item starting a separate thread ThreadPool.QueueUserWorkItem(new WaitCallback(executor.SendDelta), job);
3. public void SendDelta(object data) and within it call to PostToApi, SubmitDelta_Response rsp = RestHelper.PostToApi(_client, "connector/submitDelta", req, _logger);

4. PostToApi finally calls Execute …

   var rsp = client.Execute(request);

Here I stall.

Since the whole solution runs on my Mac (Mojave), but does stall on Linux Ubuntu 16.0.4 LTS I have been suspecting there must be something with how the kernels are doing threading. But come nowhere.