We recently started getting the exact same error along with "No Connection".
The triggering event was that the disk we use to store more or less everything got full due to a badly written script.
After cleaning up the disk everything is working except loads of "No Connection" errors aswell as this Validate and Apply Hex. UsrName() error.
We've restarted everything multiple times and googling results in no anwsers at all.
Anyone that could shed some light on this?
This is solved, after emailing with Qlik:
There is a TicketData.pgo file that can get corrupted causing this intermittend error.
There's two copied of the file, one in public_data with all the qvw:s one one in each QVS-server (just one if you don't have it clustered).
Stop all QVS (not restart, really stop them)
Delete all the TicketData.pgo files
Start the QVS.s again.
Problem went away instantly.