I generally keep 5-10 files open in Marked at a time. When exiting and reopening Marked, several of the tabs show the "Unable to Generate Preview" error.
I then:
- ⌘+title-bar-click to open the enclosing folder with the tab's file selected
- close the tab
- immediately double-click on the file
This time, the file opens in Marked correctly.
This happens so consistently that it seems like a bug rather than a transient condition that would be unique to my machine.