User blog comment:Antony1197/Super annoying courier glitch/@comment-1992639-20120622185835

You seem to have a mod that adds a quest or dialogue line to the game. As of patch 1.6 ANY MOD that adds new quests, new lines, even a NPC merchant or follower that has their own lines or dialogue no matter how short or tiny, will BREAK all dialogue, quests and interaction.

Remove or deactive any mod that adds new quests, new lines and such. This is an unfortunate change with Skyrim 1.6. All of us mod-makers hope that it is not intentional or permanent.