Go into Browser > Menu > Options > Cache Operations > then Clear History, Clear Content Cache and Clear Pushed Content... then Hit the Esc button (Just to the Right of the trackball) and it will clear those areas... and you should be able to backup everything.
And if the event that still doesn't work, try this:
Open Desktop Manager and connect BBerry, go to Backup-Restore/Advanced, open and scroll down to Browser Data Cache, mark and hit 'Clear'.....that shoul work too. Good Luck