Is that the complete response you posted?
If it's not returning a list, just this one error, then I'd call it a bug.
The call is cross-organization, in my opinion it should at least return the list of organizations your API key has access to, though I'd prefer it to retain its original behaviour and simply return the full list - that wouldn't leak any information that you could not obtain other ways through your admin access - or null out information other than the id in the response for any restricted organization(s).
This API call existed a long time before IP-based access control was a feature, I'd say it needs updating to handle it sensibly.
I'd open a support case.
Each admin is allowed two API keys, if you are only using one at the moment, a workaround would be to generate a second API key and use it for any organizations that have an IP restriction.