I've seen 3 other fixes so far:
- in .metadata/, rm .lock file
- if 1) doesn't work, try end process javaw.exe etc. to exit the IDE
- if 1)&2) doesn't work, try rm .log file in .metadata/, and double check .plugin/.
- This always worked for me: relocate .metadata/, open and close eclipse, then overwrite .metadata back
The solution boils down to clean up the .metadata folder with correct contents