Hi my name is ***** ***** get started. First, let me check over your question so I don't miss any details.
I know it sounds really weird but when this happened to me I had to download the 'Find My Friends' app even though I didn't want it, log in to my iCloud account, and accept the Terms and Conditions in the app (which loaded with no problems). After doing this, all iCloud functions were restored. Obviously you might not be able to do this on your imac due to the current issue, however you can also try from your iphone or ipad if those happen to work. Let me know what happens or if this is even a possibility for you.
Then once that is done try to see if you get the same error on your imac. You can disregard the premium services pop ups for now as you don't actually need them at this moment.
If that does not work you can also try the following: