When clicking on the dragon icon in Manolo's Tower in chapter 4 to progress the Myrtle Painting quest, I get a hint dialogue that says "<no key 'SQ315_confirmation_text' in group 'message boxes'>

In the questlog, the first entry for the "I have a message to spread!" side quest in chapter 4 reads "-<no key 'part_kill_prophet_theniran_short' in group 'SQ415'>

I'm not sure if these are base game or DLC bugs as I haven't played the game without the DLC.

Both Quests...

Myrtle and Thenrian are part of the DLC

the DLC and 1.3 are known for causing several Problems, esp. if installed to upgrade a "old save".
1.37 solved some Problems but not all, i didn't know the Bug you describe and didn't see him at the technical Forum...so..it might be the best way to send a mesage to Daedlic ;)