How historically accurate is the climactic briefing scene in Downfall?

Despite a usual number of goofs and some controversy about its historical accuracy, the 2004 German film Downfall (Der Untergang) is generally acknowledged to contain many authentic parts. Various details are obviously based on original footage and photographs, for example Hitler’s accent (in normal conversation, he did not speak with a staccato and did not roll the r like he did in mass rallies), the symptoms of Parkinson’s disease, Hitler Youth members being decorated, the Katyusha (Stalinorgel) sound, and men scavenging a dead horse. Several notable quotations are taken from reports of contemporary witnesses, in particular from Traudl Junge’s memoir Until the Final Hour (Bis zur letzten Stunde).

However, I could not find any original source for the famous scene of the briefing in the map room on 22 April 1945. It is a climactic scene for the movie and also well known as the inspiration for many internet video parodies. In this scene, it becomes undeniable that the expected relief attack of Steiner’s army detachment cannot be implemented. Hitler falls into a tearful rage and declares that the war is lost.

The incident of the 22 April is mentioned in several texts; but surprisingly, the important eyewitnesses for many events shown in the movie (for example Traudl Junge or Rochus Misch) did not actually witness this briefing. Admittedly, only a few men (some of which did not survive the war) were in the map room during the entire briefing; however, all those who had gathered at the door to the map room and in the corridors should have been able to give an account of this scene. Nevertheless, all descriptions that I have found are based on hearsay (for example Joachim Fest’s interview with Albert Speer, who arrived in Berlin on 23 April).

That leaves me with the question of how authentic this scene really is. What was the original source that was used as the reference?