[frostschutz]

It's a broken/cracked glass substrate. Impossible to fix, short of replacing the entire display unit and the display happens to be the most expensive, and hardest to reach component of an ereader so it usually almost cost as much as an entirely new one.

If you know the exact model of the screen you can search aliexpress and other sites for replacement parts, but then... it has to arrive at your location unbroken and you have to be able to perform the replacement yourself without breaking it.

eInk displays are super fragile (even more so when you take them out of their shell i.e. the ereader case)