I have this same problem. I can start a new document, but not work on it again after closing the document. If I try Word freees. Documents without Endnote codes do not have this problem. Each day I have to title the document something new and begin again. I am using Mac OS X, Word 2008, and X3. Very frustrating. I tried re-installing both Word and Endnote.
did you update your word to the latest version? A service pack may fix the problem. Its the same with endnote, update to the latest version from the publishers homepage.
Sometimes endnote crashes on my machine, when I did formate all references (appear at the end of the document). Its better to write in the “unformatted style”, both programs run much more stable.