After replacing the V9990 chip (including repairing the board with some wires because I ripped off some pads while trying to desolder it with a hot air gun) and re-soldering some cold solder connections on the other SMD chip, now it works!
I guess Simon could sell a lot more of the Kiwis, if he would sell it fully populated and tested. Hire someone for minimum wage to solder it, should be possible to solder it in some hours per board, so would cost EUR 50 more
I wish I could do that. If I would sell a ready device instead of a kit I would have to follow EMC, CE, and other regulations. This is much behind my financial limit.