After launching a variable number of RDP sessions to Windows 2012 Server R2 machines, the 3334 error begins appearing. No new connections can be made until existing connections are closed. Often, the number of connections degrades such that if the error started on the 6th connection, eventually the number of connections that will work will shrink. I've had it so that I could only have a single 2012 RDP connection open.
SSH and RDP to 2008 and 2003 servers are not affected in any way even after this error begins.
Windows 7 client connecting to Windows 2012 Server R2 with Active Directory authentication