Duplicate folder names but cannot merge

I import my images by year and month into a single catalog, matching the windows file structure.

I have two duplicate years (2021 and 2025) which contain different month sequences. When I try to move a month folder from one into another I get "Could not complete folder move: Unknown error" or "Folder already exists".  The file structure in Explorer is normal. All images are in the correct place. Any clues how I can fix this?