Deep copy the collection of at_target objects so it can't be modified while

it's being iterated

OpenSim/Region/Framework/Scenes/Scene.cs

@@ -1692,9 +1692,12 @@ namespace OpenSim.Region.Framework.Scenes
 
         private void CheckAtTargets()
         {
-            Dictionary<UUID, SceneObjectGroup>.ValueCollection objs;
+            List<SceneObjectGroup> objs = new List<SceneObjectGroup>();
             lock (m_groupsWithTargets)
-                objs = m_groupsWithTargets.Values;
+            {
+                foreach (SceneObjectGroup grp in m_groupsWithTargets.Values)
+                    objs.Add(grp);
+            }
 
             foreach (SceneObjectGroup entry in objs)
                 entry.checkAtTargets();