[stumped]

Windows defender, if used, now imposes security restrictions on folders in my documents. Try a new folder outside of there and see if problem goes away.
Or change from defender to an alternative security program.
If this is a wrong diagnosis please report back.
Are you using the menu options for export all calibre data, and then for import....

My preference is not to use that method. Instead do what previous post suggested. Copy the complete calibre library folder to external drive , using windows copy and paste, and also copy the calibre folder in app data. Then write those back to same locations on new pc