Fail importe SSL certificate

The SSL certificate previously imported into SimpleHelp has expired, and I’m currently encountering issues while importing the new certificate.
The error message shown is: Failed to load keystore.

The certificate files are valid and have already been successfully tested on other web and webmail servers.

When I import only the user certificate (without the certificate chain), I am able to access the web interface via HTTPS. However, I’m unable to connect hosts to the server over HTTPS.