​Leave it to zee germanz to completely overengineer what should have been a simple UART interface. 

I hooked up the UART <> USB adapter and tried all kinds of pinouts, it simply wouldn't work. I tried measuring the interface with the bus pirate and a multimeter but the readings didn't make sense for a UART port. 
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So ... after some digging I realized that this is not a UART port but RS485. It's a robust 3 wire connection based on voltage differentials. The upside is that it allows for very long cables, whoever might care about that in this setup.  It's also commonly used for Alarm Panels in home alarm systems. Just FYI. Also fun to play with.

Anyway... My only RS485 adapter is in permanent installation in said alarm system so I need to get a new one, or better yet, just get a USB <> RS485 interface. 

This is getting more complicated than I thought. No biggie though. Next week this should be done. I need to look into UART <> RS485 interface options if I want to control this thing from an Arduino or Raspberry Pi.