SiteCheck error: Unable to properly scan your site. Site returning error: HTTP/1.1 503 Service Unavailable
This error message refers to the inability of the remote scanner (aka. SiteCheck [1]) to read the content of your website for reasons that are out of my understanding as there could be millions of things that could explain this. The only thing that I can say for sure is, the API service tried to connect to your website and instead of a successful response it obtained a “503 Service Unavailable” error message.
Please communicate with your hosting provider to find the solution.
The same way the error suddenly appeared, the same way it will disappear when your hosting provider tells you how to fix whatever is broke in your web server. This is not a problem with the plugin nor the API service, if I have to guess, your hosting provider probably blocked the access to the IP address associated to the server that is hosting the SiteCheck service, but that’s just an speculation.
Marking as resolved, feel free to ask for more information.
[1] https://sitecheck.sucuri.net/