Does the part update records in the table or only load them?

Besides the error logs, do you have access logs where you can see how the requests are handled (timeout, no response, 500, anything else)

Nothing is performed: reading, writing, saving, update..

The first failed request of today has status code of 499.

A 499 is in the 4xx range. That is normally a client side problem. Do other people have the same problem or not?

It is also possible that it takes to much time on your server to process the request/response. Can you see if the load on your server is heavy?

Yes, other people have the same problem.

Restarting the PHP-FPM, and reloading the same page works correctly (For all the users). The CPU and RAM usage is a long way under the maximum limit, so it's not a CPU or RAM saturation.