An Exception occurred when trying to issue security token.The username and password could not be validated - SharePoint 2013

I have a test Site Collection which is using Forms Based Authentication, and site administrators cannot logon. Looking at the Event Viewer it shows the following "An Exception occurred when trying to issue security token.The username and password could not be validated"

Much time has expired on this:

1) The STS Service is running - I can browse to it! 2) The Claims Token Service is running 3) I have checked the App Pool - it is running - it has the right user

Despite all my efforts - I cannot get this to work. The only way anyone can log on is to set the User Policy against the site to Everyone with a permission level.

So, can someone please see if they can help with this because I am clean out of ideas?

Thanks