That was it - thank you. I've been migrating an app from 5.1 as well, and though I had a security domain configured in jboss.xml that file is no longer supported in AS7 - I moved the security domain configuration to jboss-ejb3.xml and the invocation succeeded.
I'm having the problem described here, too.
The security domain is set using a SecurityDomain annotation and is considered, which I know because the DatabaseServerLoginModule configured for the security domain is called.
The trouble is that the user is a UUID and not the user specified in the client. Therefore the user cannot be found.
Any Hints?