Thank you for contacting Telerik Support Team.
To be able to capture the http traffic Test Studio uses an internal proxy. What you describes seems like a proxy conflict. Do you have any active proxy on your network? An important requirement is to disable also any firewalls and antivirus tools on your machine.
You could try to capture the traffic from a new session instead of existing functional test. You could also try to capture traffic using Fiddler
to double check what might be causing the issue you are facing. The recorded traffic could be then imported
into the load test.
Please let me know the outcome of the above suggestions. I hope to hear from you soon.