First, try fixing by a rightclick, select properties and then click the repair button. That often fixes such issues.

Thanks, but there is no properties or repair option on Local Folders.

I was referring the actual folders in Local Folders. They should have properties. I agree, the Local Folders item does not have properties.

I've tried repairing the folders within Local Folders, but it makes no difference.

Just a possibility: if you have the importexport addon installed, see if you can extract the folder to eml files. If that works, you would have the messages and could then import them back. But corrupted folders are always problematic. Another option would be to restore the backup again, if available.