In order to post to Instagram plugin need to logon to your Instagram account from your server. If your server is located far from your usual login place (home area), Instagram may try to protect you and block that login asking you for confirmation. You may receive “suspicious activity” email or message and you might be asked to confirm new login by solving captcha or clicking “Confirm” button.

Please open instagram app on your phone, login with the same account and confirm your new login by either resetting your password, solving captcha or just clicking “Confirm”. It will work fine after that.