We have experienced this in the past on 2008R2 servers and have resolved it by installing: kb2661332
This is occurring on Server 2012 and I do not see any hot fixes specific to this issue.
I believe we have exclusively seen this occur to users reconnecting to disconnected sessions. New users can login fine. The disconnected users cant until the server is rebooted. It appears to be erratic as to when it occurs.
Any help would be appreciated.