Replay.Common.Contracts.Virtualization.VcenterException: 6.x version of the ESX(i) server is not tested for compatibility. Please check that you have the latest update.
ESXi version 7 is not supported by RR6.4, we are planning to add support in a future release.
We have created a patch as a workaround. Please get in contact with Support to obtain patch P-2584
If the ESXi host still shows as offline after you install the patch then proceed to re-protect it.