As you wrote:
"What could be failing to not contact Ubuntu machines in OCS Server version 2.8? Best regards."
and Agent is:
"Tue Apr 27 10:58:34 2021][debug] Ocsinventory unified agent for UNIX, Linux and MacOSX 2.8.1"
If it's true this is the problem. Try upgrade server od use Agent that is =<Server Version. I had this issue on my first install so now it's the first that I check ;) .