I also ran into this issue today, same error message. Actually tried to add vCenter 7.0 via the thick Java client, and that just failed outright. Only the web UI completed the workflow, but the error is the same. I'm a bit apprehensive to install one of these tech preview vcenter plugins, especially since I dont see anything in any of the version notes that explicitly call out vcenter/vsphere 7.0 support?
I'm running the default 6.5.0.12799420 vCenter Plug-In that came with VCO 7.6.
We're submitting a support case to confirm.