I considered / and assumed that you installed the Spotify-Application on your:
iPhone, Android Phone, and on your Computer / PC / Macintosh, right?
If you startup the Windows PC / Apple Mac / Phone application,
You did made a connection under your account, and you played some music, right?
If that is the case;
you build up data-cache, from these played music on the Phone / Computer / Mac, right?
In any installed Spotify Application (except Spotify on Onkyo, cause that is a preinstalled API),
you can log in: Go to the Menu-Bar, Look for: Edit, Preferences, Cache.
Cache is "on first use" set up to Automatic, which could me a number of % of the total ammount of your hard disk.
Unfortunetly, most Onkyo TX-NR??? do not have a hard disk - and that is where the problem lays.
1 Spotify Account, is 'in its first used' - setup on a Phone / PC / Mac, whith ??% Cache Settings.
The Accounts, remembers that, and will try to use that "??% Cache" on the Onkyo TX-NR???.
Loose these settings first, setup to the option: SIZE: USE AT MOST, add the Line-button to 0 (zero)
Therefor you: empty the cache, by closing, and re-opening the application on your PC / Mac / device.
Do this with all devices.
Close the App. Startup the App.
Check the settings again,
Go to the physical path in Explorer, mostly that is:
%UserProfile%AppData\Local\Spotify\Storage\
or:
C:\Users\%UserName%\AppData\Local\Spotify\Storage\
And delete everything "in this folder" you see here...
Close all Spotify Applications, and Spotify running Accounts, on all Devices, Computers / Phones.
I say this so explicit, because you can use Spotfy 5 times, on 5 different devices, at the same time - with a payed account.
Try Onkyo TX-NR??? ->
Set up the Onkyo with a Fixed IP Address (in Onkyo Setup -> System -> Network -> IP Address), in stead of using a DHCP leased IP-Address.
Write down the complete IP Address information, and use this 'writing' to fix this info,, by choosing;
Fixed IP Address, and set this to the Onkyo TX-NR???
Close the Setup Application, Power Off / Power On the Onkyo TX-NR???, so you know for sure that the system uses the fixed IP Address instead of DHCP.
Setup Spotify on the Onkyo again, try to login with the account info, and play the music, you could not play before.
Hope this helps, if not, let me know.