I understand others have had this problem, and nobody can seem to link it to a cause so I figured I would post it up here and maybe we can get to the bottom of it.
Account name: TheNightIsYoungAndfullOfTerrors
Character name: Remus Blackheart
The following NPC I have been unable to interact with:
Muskroots apprentice: Allows me to initiate a conversation but when I click the option to send a message across the isle it terminates the conversation.
Muskroot: Same problem as above.
Tiny(Market place): No conversation at all, he will acknowledge that i've interacted with him but no text appears.
Tailor NPC(Market place): Same as above.
No others I can recall right now but i'll add to the list If i find more.
With the most recent server update, this problem seems to no longer exist for me.