Found the issue. I have 2 boards here. Revision number 094846A0 Jove JVE0S2 etc.
The original TTN-Uno was exactly like the Leonardo. There was a protection diode that simply shorted out the power when you swapped plus and minus.
In the new version a series diode was used from the + (center) connection of the barrel connector to the input pin of the regulator. More friendly than shorting out the power at the cost of a small voltage drop.
However, on both of my boards the diode polarity is still as in the old version. Mening it just blocks all current now.
Only fix seems to be to solder it out and back with the polarity swapped.
I did a lot searching for schematics of the newer board. No luck. There is a schematic going around of an older version where the power is still similar to the Leonardo but that is clearly not matching the newer board.
Can the newer schematic be published so I can repair the board? Or can someone with access to the schematic at least confirm my diagnosis and proposed cure (reversing the diode).