I thought that DevTest might be doing something wrong with basic authentication (username:password@website), which is what I guess your "Add Participant" is doing, so I replicated that using the built-in Invoke v2 API, like this:
It works perfectly, so I had a look at my config file:
That's fine as well.
Checking the ITR log shows that username and password are displayed as expected:
So the problem must be the way you're constructing the string containing the "FTM_Password" property. I see that your error is from regex, but I don't know how regex is part of an HTTP step, so I wonder what's in your script, and what each of your steps are doing.
If you add some debugging lines to your "Script To Capture the Error Assertion" step, what is it actually doing?
Actually, what's your Excel step doing as well, as DevTest includes the use of Excel as a dataset, which wouldn't take a test step.
Finally, what is your "Add Participant" step doing? Is it REST, Web Services, HTML, etc and how is it constructing the URL?