DB settings seems an unlikely cause to me, unless you messed up access permissions (the user Q2A connects to the DB with should have all privileges on all DB tables, check with "SHOW GRANTS FOR `dbuser`@localhost;").
I'd say the most likely cause is an issue with your theme. Can you see the categories if you select a different theme? In the admin center "categories" section, do the categories appear in the HTML when you inspect the page source code in your browser?