White stains that make potatoes sticking to my iron frying pan I've got a well seasoned, never soap washed iron frying pan that I love and take care. This pan used to work well and not stick at all. Untill now.

There are now small white stains that make potatoes sticking to. Just potatoes (the last I bought, "agatha" variety), not other food. 

To correct that I tried to heat up salt (2mn) and re-seasoning (1mm of olive oil, until smoke appears, I put it under cold water and wipe with paper towels)

It didn't work, white stains are still there and potatoes still stick to.

What may I do? Do I need to wash it with soap? I don't want to do that in order to not break the seasoning, but if I can't do anything else...
These are hard-water deposits, mineral residue from your water that can build up over time, and interfere with the pan's non-stick properties.

You can get rid of them with a paste of equal parts baking soda and vinegar applied to the discoloration, and left to do it's work a half hour or so. Attack it with a nylon scouring pad, and finish off with a thorough rinse and dry. 

You shouldn't need to re-season the pan when you're finished, but a fresh coat of oil and some time in the oven afterwards couldn't hurt.