Collection was modified after the enumerator was instantiated

Hi All,

I sometimes get following errormsg in this line:
foreach (System.Collections.Generic.KeyValuePair<string, object> pair in SessionManager._stateBag)
----- System.InvalidOperationException was unhandled by user code
  Message="Collection was modified after the enumerator was instantiated."
  Source="System"
  StackTrace:
       at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
       at System.Collections.Generic.TreeSet`1.Enumerator.MoveNext()
       at System.Collections.Generic.SortedDictionary`2.Enumerator.MoveNext()
       at myDatabase.DB.CreateXmlSave() in d:\bivolino_3D\bivo\App_Code\Database.cs:line 2621
       at _Confpage.LinkButton1_Click(Objec ...