In prioritized order:

• Run EndNote as an administrator (for Windows Vista).
• Reset EndNote defaults (“Edit -> Preferences -> EndNote defaults”). This seems to work most of the time. Make sure to close Word first. After having reset EndNote, close it, and then try launching it from Word.
• The library may be corrupted. Try running “Tools -> Recover Library”.
• If all else fails, reinstall EndNote

There are other possible problems, especially when upgrading from older versions, but these actions usually work for me.

(For Norwegian readers: If you are using the Norwegian version of EndNote, the error messages will be ‘ugyldig klassestreng’ or ‘serverutføringen mislyktes’.)

“He was devoured by the deepest and most insatiable passion, which absorbs a man’s whole life and does not, for beings like Ordynov, provide any niche in the domain of practical daily activity. This passion was science. Meanwhile it was consuming his youth, marring his rest at night with its slow, intoxicating poison, robbing him of wholesome food and of fresh air which never penetrated to his stifling corner. Yet, intoxicated by his passion, Ordynov refused to notice it. He was young and, so far, asked for nothing more. His passion made him a babe as regards external existence and totally incapable of forcing other people to stand aside when needful to make some sort of place for himself among them. Some clever people’s science is a capital in their hands; for Ordynov it was a weapon turned against himself.”

From “The Landlady” (1848)