Some of our devices will not register using 'zac reg https://<our_Zenworks_server>'

Other new devices that are in the same VLANs do not have any issue. All devices (working and not working) all have ports 80 and 443 opened to the Zenworks server, confirmed via telnet.

This is the output I receive:

[root@test-device ~]# zac reg https://zenworks1

Processing Command: reg

user name:admin123

password: ********

Unable to register with the server at: https://zenworks1.

Unknown or an internal error occurred.

Error processing the ZAC command.

Has anyone experienced this before?