It started when I was trying to import photos from my camera in Darktable to an NTFS HDD (separate from SSD of OS). This is meant to be a shared drive between my dual boots (Windows 10/Ubuntu 18.04). So NTFS doesn't seem to be the issue exactly as I eventually got everything working. But then it stopped again and I was getting the same error ("Failed to import photos from camera"). I restarted and booted Windows, then restarted and booted Ubuntu, now it's working again. It looks like I may need to start up in Windows first every time I want to open Darktable in Ubuntu.
Any explanations?