Okay, found it.
Problem was not the 21:9, but rather the very low and strange resolution the game was set on the beginning.
Felt strange having to modify a .json file (%apdata%/roaming/vintagestorydata/clientsettings.json) to set the correct resolution, but at least now it works correctly even in ultrawide (3440x1440).
(and no, it did not use the desktop resolution, somewhat more of 1/6 of my desktop resolution for width, and half of that for the height)