This is a problem in with Endnote that we have had since Endnote X. The thumbnail part is not relevant. On many computers, after EndNote is installed, users find they can no longer open documents from Explorer - by double clicking, clicking on a thumbnail or by right clicking and chosing open or open with…
If EndNote is removed, the problem goes away.
You are kind of on the right track when you suggest looking in the file type information for Windows. We have linked this problem to “Use DDE”. This happens on 50-60% of the mahcines we install EndNote on. I have never tracked down the exact cause. Repairing Word (from Word Menu chose Help → Detect and Repair…) changes the error, but doesn’t correct it.
I wish that i had more information for you instead of a me too, but I can tell you that this is a problem in EndNote that has been present for at least the last three versions, and is quite a pain in the neck.
Here is what we do to fix it. I hate to recomend this because I am not sure exactly what the implacations are of making these changes.
Open your File Type editor. Tools -> Folder Options -> File types
Scroll down to “DOC Microsoft Word Document”
Click Advanced
Click Open (should be bold)
Click Edit… button
Edit the “Application used to perform action:” Go to the end of that line and remove /dde and replace it with “%1” (including the quotes).
— For mine this leaves me with “C:\Program Files\Microsoft Office\OFFICE11\WINWORD.EXE” /n “%1” (including all quotes)
Uncheck Use DDE