I had this problem as well in the late versions. I think, it's the best to go to Verdistis and talk to Trevor after you have exposed Tingalf.
The sequence is this =>
1st letter triggers Tingalf's tip, where the new lair is.
2nd letter triggers 2 speech options for Robin. The 2nd one is not very obvious, starts with "Ah, and yes..."
http://www.kiya1.de/images/Div_Robin1m.jpg.htmlDid you have both? This "ah, and yes" option triggers Trevor's speech option. I gather you had the quest to find out the truth and are a member of the merchant guild?