I just created an account on your site and saw first-hand exactly what you're dealing with.
Three places to look within:
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1.
Could you load your scripts/boardrules.inc.php and confirm that it matches the same information that is in the stock UBBT 7.5.8 boardrules.inc.php
The key I am looking at is, "USER_RULES_ACCEPTED" -- for some reason, during the newuser registration, it's not setting a date (or a "0") within your ubbt_USERS.USER_RULES_ACCEPTED database table.
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2.
Revisit: Control Panel > Registration Settings
Toggle ON "Force users to accept rules on next visit?" (at the bottom). Submit the page.
Now toggle it back OFF again. Submit the page once again.
When you have this box checked-ON, a DATE (of acceptance) is entered in ubbt_USERS.USER_RULES_ACCEPTED database table; otherwise it should usually have a "0" in it.
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3.
If you believe you've tried everything and still nothing is working for you, the bit of code that is causing the Board Rules to be displayed
again is located within /ubbthreads.php (root of your forums directory) near line 256.
// Does this user need to accept the board rules?
if ((($user['USER_RULES_ACCEPTED'] < array_get($config, 'FORCE_RULES', 0)) && $ubb != "boardrules" & $ubb != "logout") && $user['USER_ID']) {
$html->give_rules();
exit;
} // end if
