Never mind, I found the problem, in /lib/classes/database/manager/class.DbManagerMySQL.php you have a function called grantPrivilegesTo.
$stmt = Database::prepare("
GRANT ALL PRIVILEGES ON `" . $username . "`.*
TO :username@:host IDENTIFIED BY 'password'
");
Database::pexecute($stmt, array("username" => $username, "host" => $access_host));
Because my mysql server configuration requires strong passwords, using the string "password" doesn't work even when you change it in the next lines.
The code should be something like this:
$stmt = Database::prepare("
GRANT ALL PRIVILEGES ON `" . $username . "`.*
TO :username@:host IDENTIFIED BY :password
");
Database::pexecute($stmt, array("username" => $username, "host" => $access_host, "password" => $password));
Now I have Froxlor working OK.
Hope it helps,
Juan