Why does the site freeze in case of an error in the sql query, without displaying any errors?

Question

symfony

3.4 version, php 7.2, remote database mysql 5.7

This started just recently, I don’t know what could lead to this, I rummaged through everything, I don’t even know where to look.

If there is a SQL error in the request, then there is no interruption of the process, the site just freezes until it receives a 504 error from nginx, in the nginx logs there will be something like lost connection from upstream, the request logs show that the script worked fine, you can even see what kind of error returned mysql, it is clear that the script has reached the moment when it returns the response to the front. But, nothing comes to the front, the front does not see that the answer came from the back.

If the error in the request is corrected, then everything is OK, but for some reason everything freezes with an error, the front does not see the answer from the back.

Any options for where to look?