I got it working because i missconfigured something, just in case you did this too, here is my solution:
To have user-LDAP/AD login to work, you registermethod needs to be "public", otherwise you get access denied because its not allowed to create a new user. When you login with AD-Credentials there is automatically created a osTicket-User.
Hope this helps some of you.
I would recommend to osTicket-devs to still make it possible to login, even when the registermethod is set to private.