sometimes missing posts

In most cases, the site is working normally, but from time to time, namely every day, when I check 404 Monitor I see several URL with 404 Error. URL with 404 Error is /wp-admin/admin-ajax.php. At first I did not pay attention to it, but now, when I publish a post, not always, but sometimes he says “Not Found. Apologies, but the page you requested could not be found. Perhaps searching will help.”
Sometimes when I click on the post instead of the content he says “Not Found. Apologies, but the page you requested could not be found. Perhaps searching will help.”

Guys please help, I became desperate ??