Got the same error, seems like invitation service got broken =/
Good to mention that it partially "works", because I'm getting emails "Welcome to the Apple Developer Program" for every accepted invitation. Got about 22 of those in my inbox.
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Hi,
I got exactly the same problem today.
The solution was absolutely not obvious but, it happened that a new Agreement was released and I didn't have it approved. As a result I was't able to do any changes in my subscriptions.
Check under Agreements, Tax and Banking if you have a pending agreement to approve.