Sometimes, a page can show a 404 error response in Monitor Backlinks, even though it appears to have a 200 OK response.
It could be that the Monitor Backlinks crawler has been blocked. We take great care to crawl responsibly and not put any significant burden on servers, but it is of course the right of any website owner to block any and all crawlers.
To check if this is the case, try several different HTTP status checkers (there are many free ones online) and see what responses they show.
If there is a mix of 200 and 404, then it probably means that the site is blocking some crawlers, including Monitor Backlinks.
In that case, to ensure that the data is up to date in Monitor Backlinks, you can click on the gear icon for the link and then click on "Re-check" :