Try to select the entire Net before Smooth command. Altough it works for me in either case with DRC on. If you still have the problem, please contact our Technical Support.
If that doesn't work, try the "ASCII Dance" - ASCII out the board and import into a new database (don't use the same session reset as "new", close it and open a new blank session). This sometimes works even better if you ASCII-out to an older version, then import into the newer version database.