Those two folders are both provided by Explorer (Opus is just the host), so it's up to Explorer how they appear. You can probably get rid of it by turning it off in Explorer itself.
That did it, thank you.
Looks like the Windows Explorer checkbox mode doesn't behave the same as the Dopus checkbox mode. When clicking on the row of these (special folder) items (in full row select mode) it selects the checkbox too. And regards it as ready for a command (Dopus disregards the row selection if the checkbox isn't checked). When clicking the checkbox it selects the full row too. Not the same behavior as Dopus. Just as a side note, something I noticed.