I've just checked this and the issue is the type checking in aiohttp. There isn't any type checking in the request based library. That's why it is working here.
response.json() is checking, if the content-type is "application/json". For an odd reason in this endpoint the dashboard will return "text/json"
as a workaround I would disable type-checking:
change
meraki.aio.rest_session.py:Line 470
return await response.json()
to
return await response.json(content_type=None)
@John_on_API
In my opinion this is a bug in the endpoint on dashboard itself. Why is this the only (as far as we know yet) endpoint with content-type "text/json" instead of "application/json".
I could make a fix on the client and override the check in aiohttp for "text/json", but that would be a workaround which shouldn't be needed.
Edit: as posted below "text/json" isn't a correct mime type, so it should really be fixed on the endpoint
/Edit:
@PhilipDAth I'm glad that you came to the same conclusion as I =D
You've just posted yours while I was writing on my reply ^^