User permission not working after migrating SharePoint 2010 sites to SharePoint 2013

After migrating a SharePoint 2010 site to a new SharePoint 2013 platform (that is connected to the same Active Directory) using the "Mount-SPContentDatabase" command, users were unable to login.

Solution: You need to convert the users to Claims authentication! (Thx so much bart)

Once again our good friend PowerShell will do the job:
$wa = get-SPWebApplication http://mywebapp
Convert-SPWebApplication -Identity $wa -To Claims –RetainPermissions