I don't know what happened, but i tried around with the shopping cart, since i saw a similar problem there (cart was empty out of the blue).. So i removed all the cookies again and tried again with the cart.. guess what, the cart was correct, and so was the login..
This is one of the weirdest thing, and i still don't know how to explain that to the shop owner..
Anyway i guess that slipped in with a bug of an older version of Chrome, and disappeared again.. but once opened the url, it's hard to get rid of it.. it might be also a confusion with the admin, since the admin site i can access with "seasonwear.ch" not only with "www.seasonwear.ch".. anyway, i still can't reproduce the problem ..