Sometimes, Dotcom-Monitor can report monitoring errors while a target website is working properly. One of the main causes of this could be the use of automated bot management by cloud service providers. Cloud service providers, such as Cloudflare, will identify Dotcom-Monitor traffic as non-human, or bot, traffic. In this case, depending on bot management settings, all requests from Dotcom-Monitor to your websites can be blocked. Therefore, since Dotcom-Monitor will not be able to access the target website, you will receive errors for the corresponding devices.
As a rule of thumb, if you started receiving errors from Dotcom-Monitor, make sure that the website is accessible from your browser. If the website is accessible and functions properly, the first thing to do is to check if the traffic from Dotcom-Monitor is allowed in your cloud service settings. For example, if Cloudflare is your CDN provider, you can configure Super Bot Fight Mode to allow Dotcom-Monitor traffic on your website https://blog.cloudflare.com/super-bot-fight-mode/.
The best way to allow Dotcom-Monitor traffic on your website is to whitelist IP address of Dotcom-Monitor monitoring agents. See the full list of Dotcom-Monitor IPs here.