We need a little more information to help you out:
1. What version are you running? (3.1, 3.1.1, 3.2)?
2. What form factor (Tomcat, Jetty, WebLogic, etc)
3. Are you using a custom auth provider or a default auth provider on this API Project?
4. Can they include a stack trace from catalina.out or the jetty window?
5. Can you show the full URL for the server.
This is Data Explorer and how did you launch it (from LAC)? Does the Demo project work for Data Explorer?