In the scenes in 1918 Cavalry Captain von Brenken is seen walking and riding normally and freely with no issues and got insured only much later. The real Carl Friedrich von Langen was actually heavily disabled since 1915 and was stuck to a wheelchair and was able to ride only in 1920, when beginning his sport career.
The horse-riding championship in Geneva is set in October, as shown on the poster. However the actual first CHI Geneva took place in November, 10th-14th November 1926 and in later years they were done in December. It is also taken place in a hall, not outside as in the movie.
The opening credits mentioned that in November 1918 various German soldiers couldn't get home from Russia. It is very unlikely as Russia, soon to become Soviet Union declared Peace in January 1918 and the soldiers left a few months afterwards.
The movie suggests that the last horse bus in Germany was in 1919. In real life it was running until 1923.
Ernst von Brenken mentioned that he wants to join the international riding tournament CHI Geneva. The first CHI Geneva championship was in 1926, while the scene is set between 1920 and 1925 (as identified by the portrait of Friedrich Ebert, the chancellor of the Reich at that time).