Your completion block is not running on the main thread. Since UI updates have to happen on the main thread, you should dispatch the alert view to the main queue, and you'll see it immediately.
NSURLSessionDataTask *sessionDataTaskLogin = [_sessionLogin dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
if (!error) {
NSLog(@"loginWithSuccess");
dispatch_async(dispatch_get_main_queue(), ^{
UIAlertView *alertError = [[UIAlertView alloc] initWithTitle:@"Login ok" message:@"ok" delegate:self cancelButtonTitle:@"Close" otherButtonTitles: nil];
[alertError show];
});
}
}];