If your NoMachine session freezes, the best way to resolve the issue is to ssh to lts.cla.umn.edu and kill all processes that are owned by you.
ssh lts.cla.umn.edu
goldy@lts:~$ pkill -9 -u goldy
After that, you should be able to connect to lts using your NoMachine client. If you are still having a problem connecting, open a ticket with [email protected] so that we can look into the issue further and see what the problem is.