Now I can accept the invitation, but after a couple of minutes developer team disappears from developer.apple.com (it's still visible on appstoreconnect). Tried a couple of times with the same result.

Same thing here. On my account with admin access my developer team disappeared from developer.apple.com (I can still see it on appstoreconnet). I tried sending the invite to my other email, but I got the same error as above. Any solutions?