firstname.lastname@example.org. try this out.
When you get stuck in the loop, just disable the posting for Instagram for about 7 days. Then try again.
I had disabled posting to all my Instagram accounts and i just enabled one today and it started to post correctly. Will have to see how long it stays that way. Although i would recommend activating posting to no more than 1 Instagram account per week. try it out and let us know.