At some point all files in my working copy got marked with \"S\" symbol as shown below:
$ svn st
M S AclController.php
S InstallationController.php
Just a remark: I got the same S symbol when I checked out a deleted directory from the same location in the repository but using a different URL, i.e., using distinct protocols to checkout like 'svn checkout svn+ssh://user@scm.gforge...' against 'svn checkout --username user https://scm.gforge....'. I solved it by checking it out again by using the same URL that I used for the first checkout.