I have been having tagging problem with Subversion in Sourceforge. It took me several days to realize that this is not going to be resolved by itself. Once again, Google helped me found the solution:
You need to point your working directory to a different repository URL, through command like:
svn switch --relocate https://svn.sourceforge.net/svnroot/cotta/trunk https://cotta.svn.sourceforge.net/svnroot/cotta/trunk
You just need to replace 'cotta' to the unix name of your project.