@Vlad7
It's database log, not php error log.

I turned everything on

try to disable OSC_DEBUG_LOG

OK

I am not sure if it works properly for you, as this link shows empty page instead of error:
https://ukagro.com.ua/index.php?ajaxRequest=1&ajaxFindCity=1&latitude=48.1467173&longitude=17.1385448

I follow the link and see this

Last time, its database query log visible to admin only, its is not error at all.

how can i enable what you want to see?

@Vlad7
Contact your hosting provider.

The hosting provider could not determine the reason for the error 500 on the page, recommended contacting the developer 

Lol. Host do not know how to get error behind 500. Leave such host. Until you do not have error, cannot help at all.

Usually they quickly helped me, for example, on the page in the topic with banner optimization for Adsense, there was also a 500 error and they fixed it. Why they can't now I don't understand.

What file should be here?

Its no help, just tells you error. You are not asking them for resolution.

Look, I got something here after removing the "welcome" plugin

@Vlad7
You should have this on line 15 in this file:
  function wlc_param_update( $param_name, $update_param_name, $type = NULL, $plugin_var_name = NULL ) {
maybe you use older version.