I found a solution in case you have installed Eclipse(Luna) with the SVN Client JavaHL(JNI) 1.8.13 and Tortoise:
Open Eclipse:
First try to add the project / maven module to Version Control
(Project -> Context Menu -> Team -> Add to Version Control)
You will see the following Eclipse error message:
org.apache.subversion.javahl.ClientException: Entry already exists
svn: 'PathToYouProject' is already under version control
After that you have to open your workspace directory in your explorer, select your project and resolve it via Tortoise
(Project -> Context Menu -> TortoiseSVN -> Resolve)
You will see the following message dialog:
"File list is empty"
Press cancel and refresh the project in Eclipse.
Your project should be under version control again.
Unfortunately it is not possible to resolve more the one project at the same time ... you don't have to delete anything but depending on the size of your project it could be a little bit laborious.