Dear Fadi,
Thank you for your question. If you delete the two '+' from your query expression than it should work.
like:
1SET aqlGetPredecessorsForObj: ("(<\"Operation\">)DIFF(<\"Behavioral Unit\">)")
2 CC "AQL" EVAL_AQL_EXPRESSION expr: (aqlGetPredecessorsForObj) modelid: (nGraphModelID)
3 SETL lGraphNodeIDs: (objids)