Ernst Thälmann was arrested at 3rd March 1933, not before the Reichstag fire on the 27th February 1933.
Unlike in the movie the SPD party leadership was not the cause for Hitler go become the leader of Germany.
The movie shows the Nazis planning the Reichstag fire. Actually it is still not clear who actually put the Reichstag on fire. The guilt of the official committer Marinus van der Lubbe was changed since then and was cleared and put on. It is still not clear if he did it, if he had any helpers or if anyone else burned the Reichstag.
The movie shows the KPD to be a union front with the Social democrats. However until 1935 the KPD was against it which was declared in their social fascism thesis.
The first scene of the movie in 1930 shows an article of "Die Rote Fahne" about a mining disaster in the Ruhrgebiet area, which refers to the accident at "Grube Anna II" in Alsdorf in the Aachen Area. The scene in Berlin is clearly set in the spring. The event happened on the 21st October 1930. Also it caused 299 deaths and 304 injuries while the newspaper mentions 112 victims.
The band before Thälmanns pre-election speech plays the song "Einheitsfrontlied". It is also sang in the Election marching scene. The song was written in 1934 while the scenes are set in 1931/32.
In her first scene Änne Jansen receives an invitation for the Anti-War-Congress in Amsterdam. The Congress took place in 1932 while the scene is set in 1930.
During his arrest on his birthday the watchman said that more people came on his 50th birthday but not on this one. IN a following scene Thälmann is seen reading congratulation letters from Maxim Gorki and Romain Rolland. This actually happened on his 50th birthday.