Yeah that's caused by the zombie hero bug.  Basically a hero gets in a fight and dies.  But he doesn't die like he's supposed to.  Instead he's still in his ship with really low health.  If he ends up in another fight while in this state, whether by you or another empire, it causes a conflict.  Resulting in a never ending pending state which ends up bringing a quick conclusion to the game.  You either have to reload your latest save and hope it doesn't happen again.  Or give up and start a new game.  This one is a game breaker bug and I constantly have to give up and just start a new game because of it.  Usually if it's me that causes the conflict (by attacking the wrong fleet) I can get around it by just reloading a previous save and remember not to attack that specific fleet again.  If it's the AI that's caused it, you're pretty much screwed.  I just had to give up a game less than 5 minutes ago because of this bug.  I know their next update isn't until release in may, but I'm hoping they at least release a patch to fix this particular bug before than, because it's bad and it's constantly ending my games prematurely.  But I doubt that's going to happen.  This bug has been pretty persistent through at least the last couple updates.