Error Message in Endnote X5

I am using Endnote X5 with Word 10 on Windows 7 for my PhD theis. At the top of the  biblio I get the folllowing mesage:

  (!!! INVALID CITATION !!!)

However I cannot find which citation is invalid and do not want to damage the existing files by a recovery. Can you advice? I have 1249 references in my library.

With thanks.