[kh99]

Originally Posted by minifisch

Now, there is only one thing left to check for:
field5 should be unique in our database, if any one have already the same content in field5 (MC-User) there should show up a message like: "The Minecraft.net account is already in use by: <nameofuser>"

Maybe something like (goes just inside the first 'if':

$result = $vbulletin->db->query_first("SELECT userid FROM " . TABLE_PREFIX . "userfield WHERE field5='" . $vbulletin->db->escape_string($vbulletin->GPC['userfield']['field5']) . "' LIMIT 1");
if ($result)
{
    eval(standard_error(fetch_error('account_in_use')));
}

Got it to work, now there is another problem:
Could not find phrase 'account_in_use'.

Phrase "account_in_use" is created and filled with text.

What phrase type did you make that phrase? It has to be one that's loaded for that page.