The winlogon notification subscriber TermSrv failed a critical notification event.

When launching a Citrix application a user was getting a the 'Citrix launch bar' then, after a few seconds, it would immediately go away.  Checking the Application log in the event viewer displayed these entries:

The winlogon notification subscriber TermSrv failed a critical notification event. Event ID 6001
The winlogon notification subscriber Sens failed a notification event. Event ID 6004

The root cause?

A faulty path for the homedrive AD attribute:

See that 'Connect' H: To 'Path'?  That was a bad path.

Removing the attribute or fixing it so that it pointed to a path that the user has access to and exists resolved our issue immediately.