When you attempt to create a Crucible review from within IntelliJ IDEA, you receive an HTTP 500 error.
See bug PL-635.
The project open in IntelliJ IDEA is not connected with the Crucible server configured in the Atlassian Connector. So when you try to create a review, Crucible cannot find the relevant changeset or files. Crucible throws a 'not handled' exception which causes the HTTP 500 error.
Add your Crucible server to the connector's configuration, as described in Configuring the IntelliJ Connector.