I am using Spring Tool Suite(STS) 3.0.0 release (eclipse.buildId=3.0.0.201208091018-RELEASE-e42). It was running fine for a couple of months but lately my main workspace is
Solves the problem by eliminating the workspace\.metadata\.plugins\org.eclipse.e4.workbench directory.
workspace\.metadata\.plugins\org.eclipse.e4.workbench