The Modulus mismatch error occurs when a Private key generated along with the CSR code does not match the certificate issued by the Certificate Authority.
There are several possible reasons for this to happen:
- Private key originally generated with the CSR was removed from the server;
- You are not using the private key originally generated along with the CSR code;
- The certificate was issued not with the CSR originally generated with the RSA Private key;
- If you are installing the certificate via control panel like cPanel/WHM or Plesk, your control panel can automatically fetch an old RSA or certificate. You might need to paste the RSA and certificate manually after the information is fetched.
Make sure that the RSA key and certificate match.
If they do match, retry the installation inputting all the values manually. If they do not match, please generate a new pair of CSR/RSA keys on your server, reissue certificate and retry installation afterwards.