In the meantime, I have replaced most of the capacitors.
On the main PCB:
* 2x 4.7uF / 400v
* 1x 10uF / 50v
* 1x 100uF / 16v
The 470uF / 25v has not been replaced, as I did not have that one available.
On the button PCB:
* 1x 100uF / 16v
***Result:***
Unfortunately no difference. Still no response to powering up, no LED activity.
***Consideration:***
Since there is no information available, especially not about the PCB/schematics, it's a bit of guessing. So far I have replaced the relay, and most capacitors. But with no result. Both fuses measure continuity, all switches work and the heating element also seems ok.
Checking the components on both PCB's visually, there are none that show damage or any other look than normal.
What's left are transistors and IC's.
***Question:***
Any advice on my next step?