In 1916 German historian Fritz Fischer said that it was German ambition that had caused the World War 1. Do you agree, why or why not?

German ambition was a major cause of World War I. But it would be simplistic to say this was the only cause.
Very few significant historical events can be reduced to a single cause. However, the culpability of Germany stands out in this case because unified Germany was a new country at the time, having been created in 1871. The European political world was destabilized, as Germany sought to acquire colonial possessions outside Europe in competition with Britain and France. These countries, and Russia as well, feared German expansionism. Russia felt threatened by the alliance established between Germany and the Ottoman Empire, Russia's long-time enemy. Both the Germans and the Ottoman Turks wished to control the central Asian territories that were part of the Russian Empire. The Germans had an especially aggressive and ruthless imperial policy, shown by their massacre of the Herrero people in SW Africa, and later, their partial facilitation of the genocide of the Armenians by the Ottoman Empire.
All of these factors that were present before 1914 were brought to a head when the heir to the Austrian throne was assassinated by a Serbian nationalist. Though it would seem that this would have little to do with any of the countries we have mentioned so far, the opposite was true. Germany was allied with its fellow German-speaking Austrians, and Russia saw itself as the protector of the smaller Slavic nationalities such as the Serbs. But this event would not have brought on war if there had not already been tremendous tension (and preparations for military conflict) among the European countries because of their rivalries and imperialist ambitions.