Same here.I talked to the pig drunk once, but the second part don't trigger.When i drink again nothing happens.Fast travelling and restarting the game doesn' help.
I have three letters but after I read the first the option to use the letter box is gone.Restarting the game and loading a previous save doesn't help.
@ubikoality I tried the troubleshooting but the bug remains.Here is the video:
@ubikoality Correct. I can't even interact with the letter box.
I was having this problem too, now I can talk to him again.Don't know if it's related: spoiler