Like many others, I have AD as an LDAP backend for Agents.
Upgrade from 1.15.6 to 1.16.3 was smooth. Uploaded the files and the appropriate plugin versions. Logged in as an admin user and the upgrader ran. All Good.
Upgrade from 1.16.3 to 1.17 did not go so well. Uploaded the files and plugins as appropriate. Tried to login with my admin user and got Access Denied. No agent could login. Therefore the upgrader could not run. The DB was still at v1.16.3 (which turns out to be useful)
I read through this thread and went looking through the database for agents with the LDAP backend and none showed anything in the backend field of their user record in the database.
My solution was to:
Downgrade the /usr/share/osticket back to v1.16.3 and login with my admin account.
Create an extra admin account with Authentication Backend set to Local Authentication.
Upgrade /usr/share/osticket to v1.17 again
Login with the local admin account - this time the login succeeded and the upgrader ran.
Go to Agents, and select each agent in turn, set their Authentication Backend to Local Authentication and SAVE. Then set it to Active Directory or LDAP and SAVE.
Logout the local admin account, and login with my normal LDAP admin account.
All good!
Looking at the database after performing these steps I can now see that there is a value ldap.p2i4 in the backend field of the database record for each agent.
Edit:
Just one further update on this which might also help others.
Having gotten the LDAP authentication working, agents could still only sign in with their username. Sign in by email address was not working.
I found the root cause of this problem was not having specified the "Search User" and "Password" in the LDAP Connection configuration. It seems that anonymous binding is enough to find users in AD by username but, it cannot find them by their email address. Using a search user with appropriate access allowed agents to login by email address as well as username.