If you are getting the error message "Input error: k: Format of site key was invalid" then you have inadvertently saved an auto-fill data in the reCAPTCHA area under "OptimizeMember >> General Options >> CAPTCHA Anti-Spam Security" and it needs to be taken out.
This can happen if your web browser has auto-fill or auto-complete options for forms.
Please be sure unless you actually have a reCAPTCHA public and private key that these fields remain blank.
Get a FREE reCAPTCHA account through Google
If you would like to signup for reCaptcha, it is a service offered for free from Google, and you can sign up at the following URL (opens in new tab/window): https://www.google.com/recaptcha/admin
To avoid this issue in the future we would suggest going ahead with getting the recaptcha setup so you won't have to keep fixing this everytime you save settings in the general options area of OptimizeMember.