Q:

Persistent "Google sign-in" error, even with correct password

I believe this started after I installed the Google+ app, but there is a constant notification that looks like a warning sign, and it displays this:

Entering the correct password does not dispell it (it's the same password every other Google app uses, and only one is complaining).  Also, every Google app still works, each one acts as if I'm still signed in.  So where is this coming from, and how can I get rid of it? 
Update: I have tried both clearing the data for the Google+ app (which helped for about a half hour) and actually resetting my password and using the new one.  Neither worked.  

A:

Here is the solution: Login into Gmail, locate Account under the sign-out Name.  This takes you to My Account where you click on security.  Here you find a section for Application-Specific passwords at the end of the webpage.  The issue is explained that:  Some applications that work outside a browser aren't yet compatible with 2-step verification and cannot ask for verification codes, for example:
Email programs including Outlook, Apple Mail, or Thunderbird
Older Android smartphones

The root-cause of this problem is that you probably turned on the 2-steps verification without noticing this.  So all you have to do is to generate your application specific password and that is it! 

A:

I had the same problem.  My solution was to uninstall and reinstall Google+ and everything worked fine after that.  

A:

This just happened to me as well.   I had to change my account password via the website, and then my device accepted the new password. 
It only happened with the account I use for Google+, so I think your guess is correct.