Figured it out: The dongle ships with a USB bootloader, located at the end of the flash. And the saved code area is defined right there. Details here
Moved the saved code area a bit, and now everything seem to be working :)
Board file and hex attached.
(If I got the numbers right, but it's getting a bit late. Only tested with a small blinky code. Still, the 52840 has a ton of free flash, so moving the saved flash area is not a big deal)
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