Eclipse won’t start with: ObjectNotFoundException: Tree element not found

After installing some Mac OS X Yosemite updates today, my smoothly working Eclipse installation on my Mac Book Pro was broken: starting Eclipse failed with „ObjectNotFoundException: Tree element not found“ permanently!

It seems that the only thing that helps is to delete the „.metadata“ directory in the workspace – at least this solved the problem for me. But be aware of the ugly side-effect that a completely new (and clean) workspace will be created …  :-/