Sometime when I collapse a File Tree hive I will get the "windows spinning wheel" icon. It only appears in the File Tree pane. Sometimes the program stops responding, where I then have to use the Task Manager to "end task" to kill Directory Opus.
I also noticed that the Windows 10 File Explore is affected, meaning it can not display file associations and tree. This forces me to restart the explore in Windows.
That suggests it may be something system-wide, either in Windows itself or in something that interacts with both Opus and Explorer, such as a shell extension (one which generates icons, perhaps) or antivirus.