Were you able to resolve this issue? It almost sounds like there is a problem with the authentication server rather then the Tableau Server. Is this the only user who has this issue or is it for everyone?
We haven't resolved the issue yet and it is most likely due a a port being closed on the LDAP server. It is happening a development environment so is not too urgent. I will provide an update in the forum once we find the fix.
Have you found the solution for this issue? 3 years later and i'm having the same problem as you while trying to add new users.
If you, or anyone, knows how to fix this issue it would help me alot!