The ImpressCMS project had migrated to sourceforge.net when we got fed up with the low availability of the cvsdude (now codesion) service. We had had a very good experience at the beginning, but all of a sudden, major migration were done unscheduled, and because we were an open source project, we could only get mail-based support, and low-availability servers.
It turned out later that CVSDude was in the process of being bought, so in hindsight we could have seen it coming.
To be honest, the trac instance provided by sourceforge has never worked really well.
login to sourceforge shell
make a clone of your repository
create the dump file: svnadmin dump /svnroot/PROJECT/ > svndump
Managing your SVN repos with adminrepo : https://sourceforge.net/apps/trac/sourceforge/wiki/SVN%20adminrepo
loggin on to the Sf.net shell: https://sourceforge.net/apps/trac/sourceforge/wiki/Shell%20service