The 500 error points to a problem with the settings for the testing server in the site definition.
Double check the site settings, and make sure you have the testing server section properly configured.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.