The Call to Arms bug seemed to happen to me if I head right and climb down to the small group of shriekers. Slane kills the shriekers, but then the seekers start running down to the left without having a chance to speak to them. If I attack the shriekers straight down the 1st path I *think* it may complete successfully (At least Gareth speaks to me)
When getting to the ship I get the message saying 'left without the Seeker's help' and they are all dead on the ship.
I think I can go back a couple of hours to get around the bug, but that's a lot of time to lose. Hoping for a fix.