Today I removed the wheel to check what was wrong. Well… nothing 
Part 338 is not bent and still ok:
The sideway movement is actually due to the flexibility of the plastic rim:shock:
By pushing with my thumb the rim flexes by 4mm (check the edge between aluminium part and plastic part of the rim):
So this sideway movement appears to be normal on a Strida LT. I would be curious to know whether spoke wheels are stiffer than the plastic ones?
As I will order a new freewheel, I will order part 338 as well, just in case…