I don't know what it means

"PHP Error - db_select value"

Only in main.php (in admin section), and for all inactive users in online.php

That means that the script cannot select your database. What version of threads are you running and have you modified the original instllation files? If yes (on the second one) it might be an idea to restore your original files and try it again. If the problem persists, hop into infopop.com and report it as a bug.

After few days, when I wrote this post, I upgraded my forum to 6.3.2 version
Now is ok.