That matches what we see: Instagram switches these endpoints off account by account, so one account is answered where another gets a 500. Version 4.0.5 tries the other routes Instagram still answers and remembers whichever worked, so the third profile should behave like the other two.

Update the extension if your browser has not done it yet, then reload the Instagram tab. If it still fails on that one account, open the extension and go to Contact — there is a Troubleshooting box there with a report you can copy and send us, and it names no Instagram account of yours.