I know this has been posted before but I have been seeing this message pop up after some of my users try to apply a custom Stamp. The document then gets locked and the only way to remove the lock is from the management screen by an admin. 

Here is the full error from the event viewer.

The given key was not present in the dictionary.
Operation: /laserfiche/DocumentService.ashx/SaveEntry
  Message: Exception encountered, stack trace:
  Laserfiche.WebAccess.Common.Util.ErrorHandler.LogException
  Laserfiche.WebAccess.Common.Util.ErrorHandler.Standardize
  WebAccessServices.HttpHandlers.DocumentService.SaveEntry
  System.RuntimeMethodHandle.InvokeMethod
Exception details:
  Message: The given key was not present in the dictionary.
  Stack trace:    at System.Collections.Concurrent.ConcurrentDictionary`2.get_Item(TKey key)
   at Laserfiche.WebAccess.Common.Util.AnnotationUtil.ApplyAnnotationChanges(DocumentInfo entryInfo, PageInfo page, DocumentPageCache pages, AnnIDInfo annIDInfo, IDictionary`2 annIdMapping, IDictionary`2 annChanges, WebAccessSession waSession, String repositoryName, HttpContext context, List`1& changedAnnotationTypes, List`1& warnings)
   at Laserfiche.WebAccess.Common.Util.AnnotationUtil.SaveAnnotationChanges(String repoName, Int32 docID, WebAccessSession WASession, HttpContext context, List`1& changedAnnotationTypes, List`1& warnings)
   at WebAccessServices.HttpHandlers.DocumentService.SaveEntry(String repoName, Int32 documentId, Int32 curPageNum, String strCurPageId, MetadataChanges changes, GeneralChange[] generalchanges, AnnotationChanges[] annchanges)

  Session: jpngsmke

Its random and hard to recreate. Once I remove the lock and have the user sign out and back in, everything works normal. 

Web Client is at 10.4.0.309

Thanks for the help.