What are those? They look like eggs.
I don't know what those are either. What fish/shrimp/snails are in the tank?
Could be that the missing fish died and the remains have since been eaten by the other fish, but seems a likely culprit for the sudden ammonia spike, if you've made no other changes. A decomposing fish releases a lot of ammonia into a tank.
The important part for now is to keep testing daily, and doing water changes anytime ammonia or nitrites are above zero. The higher the reading, the more important it is to do large water changes to dilute it out.
I am concerned that you're still getting a reading for ammonia and nitrites even after large water changes, without finding the fishes body. A 75% W/C on a 90 L tank should have removed that 0.5 ammonia and nitrite, so the fact you're still getting a reading means something is still releasing ammonia into the water, and it's more ammonia than your filter can safely process. I'd look again, thoroughly, for the body of the missing fish. and also look into what else might be producing too much ammonia. Whether your tank is overstocked, under-filtered, over fed, or what, otherwise you might continue to find yourself playing catch up, or wake up to find your whole tank dead
Ammonia and nitrites literally burn fish, and are eventually deadly, so you want to be on top of this testing and water changing, along with determining the cause.