I would guess they use some of the standard hero's and AV's...I know from the comm fix patch, the descriptions were now too long and bugging the missions out.
I had to fix 2 of mine because reqeum was in them.
Workaround is to go in and delete the description if it is your mission.
The latest patch invalidated a bunch of arcs for some reason. Life's weird that way.
They really should be marked as "invalid, please pester the writer to fix them" or something in the search interface though.
@Mindshadow
Gotcha, thanks guys.
I almost wish my arc had been invalidated instead of changing the final climactic battle in a burning forest into ... a surreal dream of disappearing mobs and illusory matter in the labyrinthine underhalls of 'Arachnos Park'.
At first I thought it was just a fluke, but its incidence is increasing. I select an arc that looks interesting, hit 'Play' and 'Accept', and nothing happens. No contact appears, the AE emblem doesn't appear on my xp compass, nothing.
I guess it's because patches are making these arcs invalid or something, but I'm wondering if it might be something else, something with a workaround. It's hard to find arcs that yell out, 'Z, you want to play me!', so it's disappointing when I actually can't play them.