I can invoke/expose the web service in the browser without any issues, but when I try to invoke/expose it from within flash I get the error opening URL message.
Open the Web Services development panel in the Flash IDE and add your web service address. If you can view its functions and details, then it should work from your project.
If you can view your web service, but still get the error, i have found the most likely cause is incorrect variables being passed to the web service function. do a trace on your function variables to check that they are not 'undefined' or 'NaN'.