Hey Guys,
 
i just want to write it down for myself ;) Because everytime I'm searching for the workaround. 
As microsoft already confirmed the Option "Sign in as different user" is missing in SharePoint 2013 sites see KB 2752600.
Here are the workarounds: 
 
1. Start InternetExplorer  by using Run as different user <- this option can cause some other problems, so I prefer the second workaround. 
2. Access the SharePoint site by using the following url http://yoursite/_layouts/closeconnection.aspx?loginasanotheruser=true 
 
cheers
 
		    
            
	        Posted
		    
Jun 14 2013, 11:15 AM
			by
		    
Nadine Storandt