Something quite strange going on there. My guess is another program in the background is randomly triggering requests to open folders, or maybe something that Explorer would ignore but Opus tries to open if it's set as Explorer Replacement. We haven't had any similar reports.
A script add-in could also be causing it, but that's unlikely if the problem stops happening when Explorer Replacement is turned off. (Unless it was only turned off for a short time, and maybe the problem just didn't happen in that time? Might be worth a try to turn it off for longer, if so.)
You could try catching the problem while Process Monitor is logging. The logs may then reveal which software is triggering the problem and possibly shine a light on what's happening.