Every so often, and whatever I am doing on my computer, a window appears and disappears. That happens so fast that I do not see what the window is. Yet, the occurrence correlates with a scheduled task called 'CCleanerCrashReporting'. So it seems that that task is responsible. In Scheduled Tasks, the task does not show as having the 'hidden' attribute enabled. So, that seems to be the problem. I do not think that I myself disabled that attribute.
Windows 10 Pro - 22H2 (2009: 19045)
CCleaner 6.10.<this version number ought to be, and is not, copy-pasteable>. It may be relevant that I disabled the 'skip UAC' function.
(I don't know whether this has been reported before.)