In Aria Operations for logs versions below 8.16.X, the API /cluster/nodes/ returns Internal server error when the primary node is with FQDN, not IP
Aria Operations for logs version 8.12.X
Aria Operations for logs version 8.14.X
Aria Operations for logs version 8.16.X
This issue is fixed is Aria Operations for Logs 8.18.X
The recommendation is to upgrade to versions above 8.18.X