There are a few scenarios that could explain the error you're seeing:
Scenario #1
Your company operates with a split internal DNS. In this scenario, the public DNS records have been successfully updated and propagated to all servers globally. This error will be showing up only on internal networks and not on public networks (outside of your company networks.) More than likely, you may need to have your company's IT team update your internal DNS entry for the given subdomain. If this has already been updated, you may need to clear your browser cache or flush your internal DNS cache: https://www.whatsmydns.net/flush-dns.html
Scenario #2
Your IP address is currently being blocked by HubSpot firewall rules. This can happen if the source IP is attempting to access the HubSpot site/page with a high rate of requests. This can cause HubSpot security protocols to potentially place a temporary block on the associated IP. Once the behavior/rate of requests returns to an acceptable threshold, the block will be lifted, and access will return as normal. This would only affect the specific IP that is causing the high requests and will not affect the public rendering of the site.
Scenario #3
This error can occur if your DNS provider is not reliably returning the authoritative CNAME record values for the given COS site, to all regions globally. For example, some provider's servers will not 100% of the time return the CNAME record value to a requesting client in some parts of the world. More often than not, this is temporary, but can be a larger issue that you will need to resolve with your DNS provider.
