Could be many things.
For starters, which user owns /etc/sasldb2 and what permissions are set?
Are you running postfix chrooted? If so, maybe its looking in /var/spool/postfix/etc and there should be a copy of sasldb2 there?
Even if it's not using the file SASL expects it to be available and usable.
You have this line commented out in main.cf
#smtp_sasl_password_maps = mysql:$config_directory/mysql_virtual_mailbox_maps.cf
Is that where you want SASL to find the auth data? Otherwise you have no map or password file specified, and it's going to use the SASL default, which is giving you an error.