Sadly, Morrowind, Oblivion, Skyrim, Fallout 3... (pretty much any Bethesda game, lol.) is crammed with bugs and glitches that cannot always be anticipated or repaired. The easiest suggestion I can offer is a workaround: open the console, target the guard/s who is/are accusing Ulfric of vampirism by clicking on him/them while the console is open, and type "Kill" without quotation marks. If there are more than one guard attacking him, you'll have to perform the same procedure (console/target/"kill") for each bothersome guard turned vampire hunting wannabe. If they are not alive to attack him, hopefully the quest will progress as it was intended.