Jura C9 drainage valve issue (or solenoid?)

I am having some issues with the drainage valve of my C9. In about 50% of the cases in which I turn my machine on the first coffee will not come out of the dispensing sprout. All the following coffees which I make will be dispensed like it should.

I have now completely revised the drainage valve (didn't change the rubbers, but they appear to be fine) and the problem still persists.

I also checked the magnetic valve opener and that appears to work without a problem.

Am now wondering which part I should replace? Is it the drainage valve? Or maybe the magnetic actuator? Or could it maybe be the case that the pump needs replacement?

Any thoughts would be highly appreciated!

Have you checked the white cap on the plunger to make sure it is tight? It must not come off.

I am not entirely sure which white cap you mean. See below some photos of the brew unit.

See picture

Aha, now I understand. I have checked (and replaced) that cap previously for a metal version. I have already fully revised the valve, but that didn't solve the issue. I have now replaced the entire drainage valve for a new one and that solved the issue.