In some cases the automatic reconnection of the session running in a NoMachine High Availability setup (two servers in a failover cluster)  fails with:

The connection with the server was lost. Error is 5: Input/output error.

This happens when the client tries to reconnect automatically the session during the failover procedure and in the interval of time needed by the passive server to assume the active role in the cluster and take over all  the sessions. 

Improvements to the autoreconnect and failover mechanism are needed on both client and server side.