This error can occur for a variety of reasons. So, take a deep breath, grab a cup of coffee and let's begin a journey into the Bizzarro (www.bizarro.com) world of Google/YouTube logins.
First, there are two ways you can log into YouTube. You can log in directly through a browser or if you are using software like Tube Toolbox, the software can use Google's Advanced Program Interface (API). We recently switched to using the API because there are changes within YouTube that makes using it better. Of course, that causes more complexity for us (and some YouTube errors which they need to fix).
Okay, now for some pretty pictures and actual help to get you through the problem. If there is an error with your username and password, the following screen is displayed.
The numbers....
#1 - if you are using Google's 2-step verification you need to follow the instructions in this link - http://support.tubetoolbox.com/entries/21203246-fix-how-do-i-use-google-s-2-step-verification-security the steps are so involved, we did not want to clutter up this article.
#2 - if you are not using Google's - step verification click on the View Debug code and follow the instructions (see screenshot below).
#3 - If the debug code says you have "Invalid Certificate" you can click on More Options to see that you will not be able to use Tube Toolbox until YouTube fixes the issue.
If you do see "xkjllkj" in your debug code and you click on More Options, you will see the following.