Why is that when your first upload a paper onto ArXiv, Google Search shows that it has been uploaded to "https://128.84.21.199"
That number is the IP address for arxiv. Typing "arxiv.org" into your browser is effectively the same as typing that number (the domain name will get converted for you).

Why the error? The HTTPS certificate is assigned to a domain name, not to the IP address (which is good, because IP addresses may change). Even though in this case, the domain name would result in this IP address, your browser thinks it is trying to use a false certificate.

To avoid this error message, just replace that number in the URL with "arxiv.org" and the error will go away. A worse workaround would be to replace "HTTPS" with "HTTP".