After struggling with our OSTicket installation until I wanted to blow it up, I finally tried rebuilding it from scratch in a separate VM. On our previous installation, I had already tried all of the changes you suggested and wasn't sure what else to try. I imported the database from the old instance, then installed the LDAP plugin, configured it, and enabled it. It said it was able to connect to the LDAP server OK when I saved the settings, but when I tried to log in with a user from LDAP, I get an HTTP 500 error. The error_log shows the same error as I reported in my original post.In the new build I am running RHEL 7.4, OSTicket v1.10.1, v0.6.3 of the LDAP plugin, Apache 2.4.6, and PHP 5.6.32.