Bug: Ulfric e l'avanzamento Guerra civile Mantodellatempesta., Dopo neugrad fort rapporto a ulfric non possibile

This is a common bug. I'll give you multiple ways you could solve it.

1) Use Unrelenting Force to stun Galmar, then talk to him again.

2) Hit Galmar until awarded a bounty, run away and then fast travel to Windhelm to pay the bounty off. Waiting or sleeping for 24 hours, then traveling back to the camp should resolve it. If not, after attacking him again until he goes down on one knee, when he gets back up he should have the dialogue option and he won't be hostile.

3) Load a previous save before reporting to Galmar, preferably earlier during the quest.