What postwar issues caused the western allies and the soviet union to disagree?

One issue: Tensions over the East Germany / West Germany and East Berlin / West Berlin division of territory.

The Soviets tried to impose a blockade of Berlin, and the US responded with the Berlin Airlift to get supplies into West Berlin.

Another issue: How governments in Eastern Europe would be reconstituted after World War II.

The Western Allies wanted free and fair elections and democratic countries to develop, whereas the Soviet Union was looking for governments in Eastern Europe that would align with its communist system and provide a security buffer against what it saw as capitalist imperialism.

Those were two specific issues. Underneath it all was a bigger, philosophical issue: The USA was committed to capitalism and democratic institutions of government. The USSR was committed to communism and imposed authoritarian government. The Cold War was mostly a tension between these worldviews.