I had the same issue. When I sent in a blank new queryfilter it worked. Try:
DataTable table = layer.GetDataTable(new QueryFilter);
Or you use a null parameter to get all data:FeatureDataTable table = layer.GetDataTable(null);
For not selecting all data it is good to use the QueryFilter
QueryFilter queryFilter1 = new QueryFilter(string.Format("ID = {0} AND JAHR = {1}", id, jahr), true);
FeatureDataTable table = layer.GetDataTable(queryFilter1);
For selecting data by geometry:
QueryFilter queryFilter2 = new QueryFilter();
queryFilter2.Geometry = _aktGeom.GetExtent();
queryFilter2.GeometricRelationship = GeometricRelationshipType.Intersect;
FeatureDataTable table = layer.GetDataTable(queryFilter2);