Just a quick follow-up on this project. Today I switched the unit on and after a briefly turning on it shut off again, completely dead. I checked the fuses - all fine. So I had to open the case and I was presented with the smell of burnt components - but nothing really visible.

The only logical explanation: Either the DC/DC converter or the inrush current limiting resistor must have gone (or both, obviously). My money was on the inrush current limiting resistor and sure enough it reads >2 MOhms. No burn marks though. At closer inspection I found that I used an unsuitable resistor: It has a flame-proof coating, but the datasheet does not mention that it is surge/pulse rated. Oops! 

Also, the schematics showed a 33 Ohm resistor, which is a copy & paste error. I actually used a more appropriate 12 Ohm resistor - with 33 Ohms the power dissipation is needlessly high, especially at 120V. Now I have to order a more appropriate replacement...