I just finished a game that ended with a nice cultural victory. After getting to the 75%+ mark in galactic influence, I was watching the 10 weeks countdown, as I was planning to test out one last mega-ship I built on someone before the game ended. At 3 weeks to go, I received a planet from the Arceans s via cultural rebellion. After accepting this, and viewing the victory screen, I was surprised that instead of saying I had 3 weeks until victory, it said I couldn't win a cultural victory while at war.

I quickly checked the treaties relations page and it showed me at peace with the three remaining civilizations - friendly with the Terrans, and cool with the Arceans and Drenghin. Treaties did not show the red line showing me at war with anyone. So, I spoke to the Arceans and sure enough, the Peace Treaty option was available on the Arcean screen. I offered it along with a request for some BC from them, and they quickly obliged. Seems I was at war with them in some ways, but not according to the foreign relations (I was cool to the Arceans) or treaties screens.

Pretty strange...