[Konstantinos]

You insert the password in the database and not the hashed password. It should be:

pawn Code:

new buff[129];
WP_Hash(buff, 129, passwordstring);
format(query1, sizeof(query1), "INSERT INTO playerdata (user, password, Scores, Cash, Admin, Deaths, Nopm, Muted, Vip, Warn, RegisterDate, Jailed, AdminActions, Cookies, Rank, VipTime, Class, PrototypesStolen, KillAsUsaTeam, KillAsGermanyTeam, KillAsItalyTeam, KillAsJapanTeam, KillAsMercTeam, KillAsMexicoTeam, IP) VALUES('%s', '%s', 0, 0, 0, 0, 0, 0, 0, 0, '%s', 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, '%s')", pname, buff, RegDate, GetIP(playerid));

For more information about how to use Whirlpool, read its original thread.

PS: You didn't post the part that loads and checks the password.