Hi BStefan,
Is it possible that there was another local copy of the project and the page was checked out in the other copy?
To fix this, check out the page in your current project. A dialog will appear asking if you want to check out the page even though it is already checked out. Check the page out anyway which will create an "unsafe checkout". Make the changes to the page and check it in. If asked, choose to steal the lock so you can check in your changes. That will free up the lock so the page should now be checked in to the shared directory.