I would check to see if this is a document specific issue. To do this, close your document and create a brand new blank document. Switch the EndNote Web tools to the EndNote X6 tools. See if you can insert a citation into the new test document. If this works, you can try cleaning up the field codes in your original document by following the steps found in this article:
http://help.thomson.com/default.asp?portalid=TSC&article=81143
If the problem occurs with even a new test document and/or persists with your document, it would be helpful to know where you keep your library and document. Do you keep in on the local hard drive, a network drive, a cloud computing option (like Dropbox) a removable hard drive, etc.?
Jason Berman
Technical Sup Rep RS
Thomson Reuters
Phone: +1 800-336-4474
thomsonreuters.com