What’s this called, and what could be the underlying condition? It's not hypopion. I’ll post the correct answer in about three days!
Correct answer: Band keratopathy (or bandvormige keratopathie in Dutch ;-)). There's band-shaped subepithelial calcification, which can cause partial loss of vision and a corpus alienum sensation. It can be seen in chronic uveitis or glaucoma. It can also be a sign of hypercalcemia, e.g. due to hyperparathyroidism of kidney failure.