This problem occurs with NoMachine connections by NX protocol when the system is configured to use PAM Kerberos with KEYRING ccache.

The KEYRING cache is not properly cleaned up when the NoMachine session is terminated. When the maximum number of bytes allowed for the cache is reached, the user is no longer able to login to the system.

This issue has been reproduced with NoMachine v.6, but it's present also in v. 7.