Why does a 404 error occur?

Question

PHP

Colleagues, hello!
I ask you to help me understand the causes of what is happening and direct me on the right path.
There is a VPS: CentOS with a bunch of nginx + php-fpm. The VPS hosts only one IM per OS. Periodically, the store crashes from Yandex.Market due to the appearance of a large number of 404 errors.
Then (within 30 minutes - 1 hour), the store turns back on.
In the nginx logs, I see a sharp increase in the number of 404 errors over a period of time. I already open all links from the logs correctly.
What can be, in which direction to look?
There is no global load on the server at this time. For clarity, I attach monitoring graphs

Answer 1

I already open all links from the logs correctly.

Which gave 404?
What's in the PHP log?
Do you have a schedule for the number of server calls?
Can the graphics be in the same scale?