Strange DB issue after switching from Single to Multisite

Hello!
I need to switch from single to multisite. I’ve followed the steps as in the Codex, and after some errors and fixings I’ve managed it.
But I had this error after created the second site:

Database error: Table ‘tda_.wp_5_wpfb_files’ doesn’t existDatabase error: Table ‘tda_.wp_5_wpfb_files’ doesn’t exist

It has shown in the homepage the first time I’ve seen it, then disappeared (phpmyadmin was open as I wanted to delete a non-working blog I’ve created).

Do I have to expect errors? I’m not a coding expert and I’m afraid to mess up things…