I'm on a mac as well. My DLC hasn't crashed my game or stopped it from opening, my problem is that I can't tell if the game recognizes the DLC at ALL. I installed the DLC in the same folder as the base game, but when I launch the game, nothing has changed. I've done several missions in the ruins since installing the DLC (as I understand that is supposed to trigger it), but so far nothing. It's really frustrating because I don't even know if the installation worked, so I keep worrying that I'm wasting my time running quests in game to try to make it pop up.

The lastest patch, 19990 (not the DLC), made the game unable to start for me as well. I uninstalled it and then downloaded it again (through the Galaxy Client), and now it works.

Thanks for the suggestions, I'll give them a try this evening. I did try uninstalling and reinstalling, but I originally installed it using the download from the site rather than from GOG Galaxy, so maybe I'll remove everything and try that again. I put in a ticket with GOG and they had this to say:

This issue might be caused by your graphics devices not switching correctly. Please download the gfxCardStatus utility:
[can't post links here]
In gfxCardStatus set to use only one graphics device and try the game again. If this fails, please select the other graphics device and try launching again.

I haven't tried it yet, but it seems... not right. I don't know. I don't usually have to mess around on a Mac to get things working, they just work. This is the sort of thing I expect on the PC, but the PC version actually works just fine.

Darkest Dungeon is a great laptop game, though! It really kills time on planes, in hotels, etc.!

Thanks for the help, I'll update if anything works. I can't seem to find any other appearances of this issue elsewhere.