Try the “Scan and Repair” tool under the game’s options in the Blizzard launcher. Not sure if it’s the same issue that was affecting this thread two months ago, but on two separate occasions in the past month I’ve had adventures that would consistently crash when attempting to load into them, which were fixed after repairing the files.
Something strange though, these crashes have kept me out of Dalaran Heist for weeks without me thinking to try this. I then started experiencing this when attempting to resume a Blackrock Crash game. At that point I did a repair which fixed Blackrock Crash, but Dalaran Heist would still crash when I attempted to resume it. I was halfway through writing a report about it when I figured I’d try to repair again just for completeness sake. Fixed it right up and I can open it with no trouble now. But it’s very odd that Dalaran Heist wasn’t fixed at the time I did the repair for Blackrock Crash, since at that point, both of them were broken. Perhaps there have been some undocumented fixes to the files that aren’t actually being propagated?