About co-op, sometimes it can get tricky. First of all, be sure to have similar army strength, to be in same region and not to have PVP protection if other guy has.
If that doesn't work, try usual stuff like relogging and revalidating game files. Also, before you go on co-op quest, try to do co-op siege of NPC town.
GAME MODERATOR - feel free to PM me about any game related issue, bug, question...