1.9 TDI: Car Won't Start - Troubleshooting Guide

Hello forum,
I'm having a problem with my VW Golf (model 3).
"It stalls in the evening and won't start in the morning. The engine cranks, but doesn't fire. When I disconnect an injector line, no diesel comes out, and even the return line is empty."
I replaced the diesel filter and the "Knackfrosch" (likely a specific component). I reinstalled the "Absteller" (a type of valve or switch) without using any special tools, and it probably isn't related to the problem. Is it normal that you can disassemble it so easily?
Any other ideas? What is the purpose of the WFS box at the pump?

Is the error memory empty?

Is the fuel injection pump externally sealed?

Have you ever proactively installed check valves in the supply and return lines?

How do you get it to run currently?

Yes, the error memory is clear. I don't have anything immediately available regarding the check valve. But shouldn't injecting some ESP fluid directly into the system provide a temporary result?
Currently, it's not working.