The exact problem described in this nine-month-old question suddenly (and apparently spontaneously) manifested for me today. It worked as recently as a few days ago. None of the answers on that older question solved the problem. In addition, I've tried updating all my Eclipse plugins to the latest versions, and tried creating a project in a clean new workspace. No dice
Has this happened to you recently? What is the solution?
(On a side note, can anyone explain why the most popular program used by programmers to write programs is such a complete piece of garbage? Grrr...)
Edit: After attempting to fix this via all the suggested methods, I have now also lost the ability to do Team > Revert... on any file or project. Yay! Looks like I'll be getting familiar with Android Studio this weekend.