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.

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.