QNTC file system requires digital signing.

We just upgraded to IBM i V7R3 and suddenly we are getting an issue when
trying to connect to /QNTC locations:

QNTC file system requires digital signing.

Message ID . . . . . . : CPDB053
Date sent . . . . . . : 01/09/17 Time sent . . . . . . : 10:22:06

Message . . . . : Error exchanging security information for user
DOCULEX400
on network server DOCULEX13NY.

Cause . . . . . : The IBM i NetClient (QNTC) file system has encountered
an
error when authenticating user DOCULEX400 with a network server
DOCULEX13NY.
Recovery . . . : Ensure the following:
- The user is set up on network server DOCULEX13NY.
- The IBM i user password matches the network server user password.
- Network server DOCULEX13NY is enabled for digitally signed
communications.
Technical description . . . . . . . . : An error has been detected while
the
QNTC file system was exchanging security information with a network
server.
The error class was 0, and the error code was 3401.

Possible error class and error code values include:
Class 0 - QNTC file system specific security error.
1 - QNTC file system requires digital signing.

What did I neglect to do?

Paul