I had this bug. I have a screenshot, but it is important to note, to fix this issue you MUST EXIT THE GAME. As a computer programmer by trade, I have also taken the liberty of adding a bit more information to make this (hard to reproduce) bug easier to debug.
* I too was in a holy war (specifically, I was defending a holy war as Aragon, against the Sultanate of Africa)
* I was approached by two brothers of the faith to join the holy war. I declined as I wanted the prestige. These people were the King of Andulisis (sp.) and the King of Aquitine (sp.)
* Other people were hostile to other people. While I didn't reload as another character, I saw the Ummalyds (sp.) seiging some obscure territory, despite also being at peace.
* I was NOT hostile to EVERYONE. I spent about 5-10 minutes looking through people, and to my surprise, I was not at war with any holdings DIRECTLY CONTROLLED by the King of Andulisis. I was at war with all holdings held by his vassals, however. I was at war with all holdings controlled (whether directly or indirectly) with the King of Aquitine. I was hostile to everyone else I could find, even the Indians.
* Italy was hostile (diplomatically) with some minor Scandinavian counts from raiding. Is it possible to get to the Kingdom of Italy easily via rivers as Sweden? Probably unrelated, but was odd nonetheless.
* Saving and reloading old saves WITHOUT EXITING THE GAME lead to me being hostile to all other people. This happened even if I loaded a save 20 years ago, where the bug was definitely not present. This implies that the issue is probably something that is stored in memory between games, but NOT stored in the saves.
* Exiting the game (to Windows) and reloading the same save fixes the issue, and I am not at war with anyone.
This is a serious issue ... what we need is to:
A) Make it reproducible. My best guess is that it is caused by denying people the right to join holy wars, as that is the 'oddest' thing that happened.
B) If you encounter the bug, and you have not exited the game, then try saving, and reloading as ANOTHER CHARACTER. Are you still hostile to everyone
C) If you encounter the bug, try loading a completely new save, with a completely new history. Are you hostile to everyone in that history too?
I have autosaves from a period about a year before the end of the holy war ... I will try to make it a real save and then try to reproduce the problem this evening.