You’ll usually see this error if your combined API key in your WHMCS installation doesn't match that originally generated in your 20i account. This can happen if you process a password reset for your main 20i account, as this will also reset the combined API key. You'll need to obtain the combined API key. This can be found within your 20i Reseller account by going to Reseller Management -> Reseller Preferences -> API, or by going here: https://my.20i.com/reseller/api. Then, within your WHMCS admin, navigate to Setup -> Products/Services -> Servers. Locate server details a little further down the page and then enter in the combined API key as the password. You shouldn’t now experience the '401,“message”:“Invalid Authentication”' error.