Greetings from Sydney, Australia. We have a similar issue in Australia with a pretty warm climate in Sydney. It seems affect the rubber moulds that surrounds the glass on all windows in my 320i.
A VW mechanic friend out here suggested a silicone spray to the rubber mouldings from time to time, is a good way to deal with glass sticking when windows going up and down.
I completely busted my driver side window six months ago. The motor still works fine, but was 'slow-slow', really laboring and doing it tough when I was closing (raising) the window. I heard this large 'bang' - and the glass started coming out at all sorts of weird and odd angles. I literally had to wrestle the glass back into position physically. The window hasn't been up or down since.
It's only gone into the mechanics today to get fixed.