Hi Daniel,
Thanks for your answer, but:
- the HDMI to SDI 12G is brand new, I don't have the front panel, and I didn't load a LUT
- the Teranex AV behaves exactly the same, but it works if I use an AJA Hi5-12G to convert back the SDI to HDMI
- I rebooted on Windows, no problem
So, it seems to be a screen calibration problem on Ubuntu. I have no experience with that, time to try I guess

EDIT: well, if it was a monitor calibration issue, the AJA wouldn't work either. Still giving it a try, but I'm a bit lost.
EDIT 2: calibration fixed the problem. Of course now I get wrong colors with the AJA, but that's not a problem as I'll use either one or the other. Still, this might only be a workaround to a deeper problem.