diff --git a/UTILS/Exchange/ms_graph.js b/UTILS/Exchange/ms_graph.js
index 187c45f4c5..fe6cfd9dbb 100644
--- a/UTILS/Exchange/ms_graph.js
+++ b/UTILS/Exchange/ms_graph.js
@@ -441,10 +441,15 @@ function updateExternnr2(userPrincipalName, id, hostPrincipalName)
     var organizerCalendarEvent = getCalendarEventByICalUId(hostPrincipalName, roomCalendarEvent.iCalUId, roomCalendarEvent.seriesMasterId);
     if (!organizerCalendarEvent)
         return "";
+    if (roomCalendarEvent.type == "singleInstance") {
+        id += "|" + "|%";
+    } else { // occurrence of exception
+        id = "%|" + id + "|%";
+    }
+    id = id.replace(/_/g, '\\_'); // Escape de underscore
     var sql = "UPDATE res_rsv_ruimte"
             + "   SET res_rsv_ruimte_externnr2 = " + safe.quoted_sql(organizerCalendarEvent.id)
-            + " WHERE res_rsv_ruimte_externnr LIKE " + safe.quoted_sql(id) + "|| '%'"
-            + "   AND res_rsv_ruimte_verwijder IS NULL";
+            + " WHERE res_rsv_ruimte_externnr LIKE " + safe.quoted_sql(id) + " ESCAPE '\\'";
     Oracle.Execute(sql);
     return organizerCalendarEvent.id;
 }