Although I didn't modify any settings, no icons appear to the left of files. Strangely, they seem to appear for about a second sometimes, but in all cases the display is left with no icons. What do I do?
As a quick first test, try running Clear Icon Cache in case the cache is corrupted.
If that doesn't help, please try using ShellExView to disable icon-related shell extensions. Start with DropBox and any others related to cloud storage, as it's usually them.
After disabling shell extensions, fully exit Opus (via File / Exit Directory Opus) and then restart it. (Or, if in doubt, reboot.)