You may be receiving this error as you have replaced your CMS installation on the same URL and will now have incorrect client information. If you DM us the URL we can reset this for you.
For those with the error message containing “API configuration problem, consult your administrator” this error will have a companion message in your CMS logs which will give more detail. It will possibly be something to do with how the CMS has been configured with encryption keys or RSA public/private keys.