VNC connection for LDAP user

We recently installed RHEL 7.3 , migrating from RHEL 6.8 and our many users work thru VNC on this server.

1. If I create a local user, then VNC works OK
2. When an LDAP user creates VNC, apparently the creation succeeds.
3. When trying to connect to the newly created VNC we get black screen and the cursor as a dot, seems that GDM doesn't start
4. If we add the uid of the LDAP user to local /etc/passwd, group, shadow the VNC opens OK, that looks like an ugly workaround.

I suspect something related to authentication, but could not pinpoint the issue or find similar problem reported on this site.
Looking at other posts I saw some suggestions related to changes in /etc/pam.d directory and some posts on web related to joint
PAM and LDAP authentication, but nothing concrete specific to our observed problem.

I'll be glad to hear if others connected LDAP users to VNC and what needs to be done.

Thanks,
Avi Efrati efrati@eng.tau.ac.il
VLSI Lab Manager, Tel Aviv University