Disclaimer: I don't use DPP4, but am definitely familiar with macOS as well as software on that platform. So here's a possible fix.
- From Finder, hold down the option-key and while that is pressed, click on the Go menu and then select the "Library" menu item. That menu item only shows when the option-key is pressed. A Finder window for "Library" should then appear.
- There's a subfolder inside "Library" named "Preferences". Try to find either a file or folder that starts with "DPP" or perhaps "com.canon".
- Right-click (or control-click on that item and select "Compress <filename>". This will create a ZIP file (basically you're backing up the original data).
- Delete the folder or file that you right-clicked on in step #3.
- Relaunch DPP
My thought is that the application preferences have become corrupted and this should thus also clear out any "previously opened windows" list.
If this doesn't fix the problem, do the following to reverse the above:
- Delete any new preferences file that DPP may have re-created (again look for any file/folder with "DPP" in the name or something like "com.canon".
- Double-click on the ZIP file you created in the earlier steps. This will re-create the original preferences.
- You can now delete that ZIP file.
Perhaps the application itself is corrupt. So try uninstalling and re-installing the DPP software.
Beyond that, it could be a corrupt data file which could be just about anything. Sometimes that file would be somewhere in "Application Support" inside of that "Library" folder. Or could be corrupt files say in your Documents folder. So if you have backups made (that would be backing up at least the data files that the software uses) you could try restoring those files.
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Ricky
Camera: EOS 5D IV, EF 50mm f/1.2L, EF 135mm f/2L
Lighting: Profoto Lights & Modifiers
