I do not know the what or why, but I have a very vague idea.
In addition to the main “home” server there are intermediary “proxy” servers that serve cached content that is not necessarily up-to-date with the “home” server. I would assume that the proxy servers perform periodic updating frequently, at least I would hope so. In other words, if a proxy server hit the source server during a problem that proxy server would return error headers when the domain was requested, while other proxy servers would still serve the cached content and not errors.
I think the problem could be one of two or both. First, the main source server would need to let the proxy servers know there was no longer a problem with the domain. Second, the proxy servers would need to fetch and cache content from the main source server.
If the main server does not make the fact a problem has been resolved discoverable, and / or the proxy servers do not check to see if the problem has been resolved, those proxy servers may continue to return error headers for requests to the domain.
Of course it could be a problem with the proxy server itself. But as there are so many I believe that is unlikely in this case.
It didn’t seem appropriate to me, given the category description:
Domains
Talk about domain registrars and how to secure the perfect name for you or your company.
I also considered Server Config, but as whatever is happening seems to be outwith the OP’s control, I settled for here.
@info2108: I sent you a PM yesterday, asking what information you would like added to the first post. (I was trying to keep the off-topic exchange out of the thread.)
But it doesn’t give the whole picture. https://www.uptrends.com/tools/uptime consistently shows that the site is available in London, Edinburgh and Dublin, yet I remain unable to access it from the west of Scotland. It does not seem to me to be a simple issue of working in some countries but not in others.
Some chap from another forum called Rotary Steve guessed what the problem was relating to, so I simply enabled my DNSSEC on my domain name and it fixed it pretty much straight away…