That version is quite old, you should contact your server admin to upgrade to at least 5.3. Anyway, as a workaround until your server is upgraded, you can do a manual edit
File xenforo/library/bdTagMe/Engine.php
Line 297
PHP:
$posOpen = self::utf8_strripos($message, '[URL', $position - utf8_strlen($message));
Change it to
PHP:
$posOpen = self::utf8_strripos(utf8_substr($message, 0, $position), '[URL', 0);
Also, on the same file
Line 323
PHP:
$posOpen = self::utf8_strripos($message, '[PLAIN', $position - utf8_strlen($message));
Change it to
PHP:
$posOpen = self::utf8_strripos(utf8_substr($message, 0, $position), '[PLAIN', 0);