Thanks @NiLuJe for the help. I made a quick fix to get it working on the Sage, though it will probably break other devices.
For anyone interested, you can can download the fixed libkobo.so from here:
Put it in
Code:
.adds/qt-linux-5.15-kde-kobo/plugins/platforms
after making a backup of the old one.
Please let me know if it works on your Sage or if it broke your working device.