OS Catalina Version 10.15.1 (19B88) problem - Cannot open files stored on the desktop or in the Documents folder

I use a MacBook Pro (15-inch, 2018) with OS Catalina Version 10.15.1 (19B88), and recently have been experiencing a frequently reoccurring problem - I cannot open any files, e.g.: xlsx, docx, pdf, Numbers, Pages, etc stored on the desktop or in the Documents folder, unless I restart the Finder.

Files in other folders, such as Music, Pictures, etc. can be opened without any problem.

Also, saving a file from iCloud on the Mac desktop gives an error: " The operation can’t be completed because an unexpected error occurred (error code -8003)".

Any help would be much appreciated...