Looks like this issue also affects me.
When trying to log into a disconnected session I get the error message: “the session negotiation failed”.
I am trying to connect from Mac OSX to Ubuntu Linux Machine. In my error log I have the following on the last line:
NX> 595 Cannot open the file ‘/home/[username]/NX/var/db/limits/sessions.[username]’ for reading: 13, Permission denied.
I suspect it has something to do with a pending / automatic update in the background because I remember a notification on the previous session log in.
Is there any way to solve this issue remotely?