Why was the Iron Curtain a problem?

It prevented the Allies from knowing what the Soviets were up to.

The Soviets were participating in the Cold War.

It blockaded access to Berlin.

It separated North and South Korea.

The Iron Curtain was a problem because it blockaded access to East Berlin and prevented the free flow of Germans across the borders between East and West Germany. More broadly it prevented the movement of ideas and peoples across this artificially created boundary. And more importantly than that it signified the dividing line of the Cold War between Western Europe and the United States and the Soviet Union which had a major impact on fomenting mistrust and conflict.