Why does my Shimano Nexus 8i need readjustment after every gear shift?

Since a month or two, almost every time I change gear my Shimano Nexus 8i becomes "unadjusted". I have readjusted it dozens of time but the problem persists. The gear is really easy to adjust (set it to fourth gear, adjust two lines so they line up by screwing a "nut" on the lever) but as soon as I have shifted the gear a couple of times the lines no longer line up (they end up being wrongly adjusted in both directions).

What can cause this? Dirt in the shifting cable (Is that the correct term? The "hose" that the wire runs into.)? How to fix?

I have seen this question but that situation is different. I can adjust using the barrel adjuster (which didn’t work for Pedro posting that question), OTOH, my misalignment appear immediately after adjusting (which it didn’t for Pedro).