When Assembler was installed, was security turned on or off during the configuration manager portion of the installation? My guess is that security was turned on, in which case you'd need to call
Context ctx = asm.login("username","password");
first and then pass that context in to the invoke method, as in:
ASMoutput = asm.invoke(DDXDocument,ASMinputs,environment,ctx);
To quickly verify the security settings, find the adobe-Assembler7.ear that was deployed, and unjar it. Then unjar the assembler7EJB.jar that was extracted and look at the contents of the security.properties file. If security.groups in that file is blank, then security is off, and if it an asterisk "security.groups=*" then security is on.
If security is off and you still get that exception, then it might be time for a call to tech support.