After around 200h of game play I finally decided to join the Legion and take out the racist Ulfric (I am an elf...).
However, after completing The Jagged Crown (and being very suspicious of why any of the Stormcloaks weren't attacking me) I started Message to Whiterun. However, when I go to talk to Ulfric, he completely thinks I am a Stormcloak and the next mission starts as If I were. What is happening? How can I make clear that I am not a Stormcloak? A polite mail to Ulfric is not possible, so what should I do?
NOTE: this may be the reason of the bug: Long ago I joined the legion (but didn't want to) so I reset the mission with commands. Later, to restart it I also needed to do it with commands, starting The Jagged Crown directly.
Additional information: I tried setting the stage of
Message to Whiterun to different objectives. The best I did was to actually reach Whiterun and talk with the Jarl, but then the general leading the defense doesnt want to further advance the mission. I can't either start the siege of Whiterrun by command lines, I don't know why.