Answer
This is typically a TLS/certificate trust issue, not an Add-in configuration problem — check whether the IFS Cloud environment is using a certificate signed by a trusted public/enterprise Certificate Authority, or a self-signed certificate; Office's Add-in security model blocks content from an endpoint whose certificate isn't trusted by the client machine, which is exactly the 'not signed by a valid security certificate' message. Switching the environment to a properly CA-signed certificate (or ensuring the self-signed certificate is explicitly trusted on the client machines running Excel) resolves this class of error.