Sorry for the necro, but hopefully this helps someone.
I had this exact same issue -> Wulbren was hanging out at a table with the other two deep gnomes and was only spouting that same line at me. Barcus was standing 10 feet away and the only thing he'd say to me was something along the lines of "Not now, Wulbren is here!" The quest said to talk to Barcus, but that wouldn't advance anything.
THE ISSUE: Barcus HAS to speak with Wulbren before anything can advance and the game bugged out, not allowing either of them to get close enough to talk to each other.
THE SOLUTION: Have Karlach chuck Barcus at Wulbren or vice versa.
I found someone mention this on the bg3 subreddit and it totally works. Throwing them close together initiated a conversation between them and then allowed me to talk to Barcus to complete the quest and get the next part from Wulbren.
You will have to deal with some attitude for "harming" someone in front of the Harpers (and property damage if you accidentally break a chair or a table), but Karlach is more than adequate to talk her way out of it using an intimidation roll.
