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Hi Gerrit, can you describe the situation a little more? Were the leaves bent over and laying in water / muck / on the ground? Were they still standing upright? Dry or wet? Was the larva sandwiched between leaves near the base of the plant, or on a single leaf higher up and out in the open? Was it on the leaf or inside the leaf? etc.
The larva's overall appearance reminds me at least vaguely of some apparent chloropids I've found feeding between leaves at the base of sedge (Carex sp.) plants, but those larvae usually have a couple of fairly obvious projecting lobes on the posterior end and I'm not sure I'm seeing that in your larva.