To enable a debugging log, nginx needs to be configured to support debugging during the build:

./configure --with-debug ...

Then the debug level should be set with the error_log directive:

error_log /path/to/log debug;

The nginx binary version for Windows is always built with the debugging log support, so only setting the debug level will suffice.

Note that redefining the log without also specifying the debug level will disable the debugging log. In the example below, redefining the log on the server level disables the debugging log for this server:

error_log /path/to/log debug;

http {
    server {
        error_log /path/to/log;
        ...

To avoid this, either the line redefining the log should be commented out, or the debug level specification should also be added:

error_log /path/to/log debug;

http {
    server {
        error_log /path/to/log debug;
        ...

It is also possible to enable the debugging log for selected client addresses only:

error_log /path/to/log;

events {
    debug_connection 192.168.1.1;
    debug_connection 192.168.10.0/24;
}