In my case PLC code generation failed due to the concerned subsystem's input and output ports not being connected.
However, the issue of unconnected ports was only displayed as "warning" in the diagnostics window making me think specific diagnostics settings are required for PLC coder related debugging.
Yet, since the generation failed exclusively due to the subsystem's external inputs and outputs, changing this diagnostics setting could also cause uncrucial errors to display.
Conclusively, regarding the intention of my question, generally no specific diagnostic settings have to be adjusted in order to debug PLC code generation.