Well, everything I posted is likely irrelevant then (you did post versions, didn't you)!
On that note, I supposed I'll go try to play with vROPs and vRA now (8.5 and 8.5), though we only have a single tenant, so again I may not find anything relevant...
"vRealize Operations Manager 8.2 or later supports one-to-one integrations with vRealize Automation 8.x, you can integrate one instance of vRealize Operations Manager 8.2 or later with one instance of vRealize Automation 8.x"
So, it appears the Endpoint guess was correct. Does the user you're trying to use have "has both organizational owner and Cloud Automation Services administrator permissions."?
Together these read as if you need to use a single account to connect and monitor the entire vRA as one, and not each tenant separately.
If you had a use case where you're trying to have USERS of the tenant monitor their Tenant through vROPs, you'd need a separate instance of vROPs for each tenant (and given the permissions they require above, I'm not sure how that would work out in practice anyway).
"vRealize Automation 8.x or later supports one-to-many integration with vRealize Operations Manager 8.2 or later, you integrate more than one vRealize Operations Manager 8.x instance with one vRealize Automationend point."