Nothing yet, I'm still travelling and don't fancy dismantling my case (knowing my luck, ...)

The good news is that drying out the wheels seems to have improved the situation, so it's not as pressing, and can probably wait until I get home.

The issue seems not to be with the axle of the wheel (this is smooth and it rolls well), but with the vertical axle (this is still stiff and made the case hard to turn).

What I think I will do is remove the whole wheel assembly using a Torx driver, clean up the vertical axle and reassemble. Then I'll report back (or, as I suspect, ask for extra help/the address of my local service centre).