ESXi 6.5 is not supported on HPE Proliant BL460c G6. The latest version of ESXi supported on BL460c G6 is 5.5U3.
You can try and update the firmware and iLO on the server.
Are you using the HP ISO which includes HP agents/drivers? It might be worth updating/downgrading those.
Yes, I know they are not officially supported by VMware however they worked right with the version VMware ESXi, 6.5.0, 13635690 (6.5U2) and last update to 6.5U3 13932383 produced the false alarm. If the only problem that appears is this false alarm, I don't want to rollback to the previous version because in the cluster we have more modern and supported-hardware without errors and I would like to mantain the same version in all the ESXi servers that make the cluster.
For these old servers, we use the official image of VMware instead of HPE custom image because the last one raised errors when we tried with these servers. I found minor updates of iLO2, so I will try to update iLO2 and check if the alarm disappear.
We plan to replace this old servers with new ones at the end of the year but in the meantime I need to mantain them to provide more power to the cluster...