Thank you for your response. I already had that checked, but it appears I have it working now. Full disclosure I am still running the following...
osTicket Version v1.15.1
Web Server Apache/2.4.6 (CentOS) PHP/7.3.26
MySQL Version 5.5.68
PHP Version 7.3.26
Here is the current plugin configuration that appears to be working for Agent and Client logins:
Default domain: server.domain.org
DNS Servers: DNS IP
LDAP servers: server.domain.org
Search User: domain\aduser
Password: pw for domain\aduser
Search Base: DC=server,DC=domain,DC=org
LDAP Schema: Microsoft Active Directory
Staff Authentication: checked
Client Authentication: checked
Note that I added the AD server to the search base. I'm unsure why this config works now since this is a setup I tried previously to opening this thread. Glad its working, will make a huge difference for us!
You are welcome to close this thread.