i think you are missing a hierarchy here, that is why the error "Error: System.err: java.lang.NoSuchFieldException:zeilenOid "
you have ..
long zOid = (long) lr2.fieldGetter(resultObject, "super.lnkMatrix.lnkZeile.zeilenOid");
shouldnt it be in
"super.lnkMatrix.lnkZeile.zeilenOid. "smonitor.model.listenverwaltung.matrixZeile"?
zeilenOid is inside "smonitor.model.listenverwaltung.matrixZeile".
not sure about the java over http protocol , but there are generic functions which might help you
lr_xml_get_values("XML={Response}",
"FastQuery=/super/lnkMatrix/lnkZeile/smonitor.model.listenverwaltung.matrixZeile/zeilenOid",
"ValueParam=zeilenOid",
LAST);
value should be available in parameter {zeilenOid}