Answer:
The issue which Mr. Kraler want to discuss privately with Mr. Frank is D. workman he suspects knows about the Secret Annex
Explanation:
Mr. Kraler really wanted to talk to Mr. Frank privately. Mr. Frank even told him to speak in front of all. They both find out that one of the Workmen is blackmailing Mr. Kraler about the secret annex who stares at the bookcase. Mr. Frank even pays that half amount to keep his mouth shut. They can't do anything else but wait for his next step that what he'll do. This is the whole conversation between Mr. Frank and Mr. Karler. After this Mr. Kraler leaves the room.
