krohnjw,
to me, there might be two reasons:

- the most obvious: connectivity issues (firewall with DROP policy, etc) that make the UI freeze (UI thread is the same of Excel RTD thread, at the moment)
- more sneaky: an uncaught exception is generated at:
Code:
new ThreadStart(delegate() { lsClient.Start(pushServerUrl); }).Invoke();
Try to replace it with something like:
Code:
new ThreadStart(delegate() { try { lsClient.Start(pushServerUrl); } catch (Exception e) { flowForm.AppendExcelLog(e.Message); }; }).Invoke();