Attempted to lock an already-locked dir: Subversion issue

"Attempted to lock an already-locked dir" - svn: Working copy locked; this message is frequently faced by users of SubVersion (SVN) source repository. The message is self explanatory; simply some source files are locked and no commit or update operations allowed on those files/folders. In some occasions, Software developers in a team receive this message in a SVN operation even though none of them have locked the files or folders; causing confusion. So how would you try and resolve?

We use Subversion to create and store our projects in repositories; Subclipe (an Eclipse plugin) as the client tool to connect to repository.

We tried following solutions

1. Commit Changes - change and commit changes into repository
2. File Delete - delete the files and commit the delete into repository
3. Override & update - override the local changes and update from repository

None of the above options resolved the issue, but got the same type of error messages. Followings are the error messages shown inside Eclipse while trying the above mentioned options.

commit -m C:/workspace/MyProject/config.properties
    Attempted to lock an already-locked dir
svn: Working copy 'C:\workspace\MyProject' locked

update -r 13 C:/workspace/MyProject/src
    Attempted to lock an already-locked dir
svn: Working copy 'C:\workspace\MyProject' locked

delete --force C:/workspace/MyProject/config.properties
    Attempted to lock an already-locked dir
svn: Working copy 'C:\workspace\MyProject' locked

Use Cleanup command

Subversion supports a command named "Cleanup"; it is used to release the locks on a project.

If you have faced with the above issue while no one has locked the project files, you can run this command and resolve the issue.
You do this in Eclipse by right clicking the project; then Team->Cleanup as shown in the image.