QueryDef with SDE (FeatureQueryClassName)

Hi!
I'm using a queryDef to join two featurceclasses and built a new featureclass. Later this new featureclass is seleted by queryfilter and the selectionset is used to make a featureselection. So I can select all objects from featureclass1 that are joined with feaureclass2. With MDB everything works fine.
Now I have to use SDE-FeatureClasses in an version an I get an HRESUL-Error. But I don't know why. I already put the dbname and owner (qualifier) in front of the fc-names.

Here's my code:

IQueryDef queryDef = ws.CreateQueryDef();

//Setting the SubFields, Tables, and WhereClause:
//Single table with a WhereClause
queryDef.Tables = "dbname.dbowner.ele_kv, dbname.dbowner.ele_stat";
queryDef.SubFields = "dbname.dbowner.ele_kv.shape, dbname.dbowner.ele_kv.objectid, dbname.dbowner.ele_kv.stat_ref, dbname.dbowner.ele_stat.objectid");

queryDef.WhereClause = "dbname.dbowner.ele_kv.stat_ref = dbname.dbowner.ele_stat.objectid";


//Abfrage als FeatureClass erzeugen
FeatureQueryNameClass qfcBM = new FeatureQueryNameClass();
qfcBM.WorkspaceName = ((IDataset)ws).FullName as IWorkspaceName;
qfcBM.QueryDef = queryDef;
qfcBM.PrimaryKey = dbname.dbowner.ele_kv.objectid;
qfcBM.ShapeFieldName = dbname.dbowner.ele_kv.shape;
qfcBM.Name = "QRY_KV";
qfcBM.CopyLocally = true;

//DOESN'T WORK!!
IFeatureClass fcQuery = qfcBM.Open() as IFeatureClass;

Can someone help me please?
Thank you!
Andrea