I don't think its the '.' causing the problem. Is CiscoWorks installed in this directory because if so you cannot backup to the install directory.
It also should not be backed up on a system directory like /var or /tmp
You can possibly try creating a dir like /opt/backup and making sure casuser has write permissions.
If you look in the dbbackup.log or daemons.log you'll prob see a more specific error like:
INFO: Backup cannot be taken on /cs/CW2000/CSCOpx since CiscoWorks is
installed in this directory
INFO: Please try again