Reading on, the problem seems to be not really specific to the windmills themselves, but to the fact that Germany is transmitting a lot of power from the North

Reading on, the problem seems to be not really specific to the windmills themselves, but to the fact that Germany is transmitting a lot of power from the North (where the windmills are) to the South (where the industry is). There is also a lot of transfer between Austria and Germany going on (with Austria's hydroelectric pumped-storage power stations essentially serving as batteries), and the Czech's are unhappy that their grid is used for this transmission for free.