The View Mode is Details. The problem is with any file (single click won't open it). FWIW the Windows Folder Options-General is set to single-click to open an item and the Windows Explorer works that way. It's just DO that won't hold the setting.
It's strange if the option is on in Preferences but not taking effect in a Details mode lister. (Explorer is used to display a handful of folders Opus doesn't handle natively, which is another factor, but since you also have the option on in Explorer we can rule that out.)
Next time it goes wrong, please check that the View Mode is still Details for the file display that isn't doing what you expect. It's possible that something is changing it into Power Mode without you realising (it can look identical to Details mode, depending on configuration).
FWIW, I have this same problem with Directory Opus 11 Pro Edition (build 5177, 3/5/2014 12:32:18 PM), which I am now using in trial mode.
As a reminder, I'm running it on Windows 8.1 (64-bit).
I sure would like to solve it; DO has never annoyed me like this before...