In the directory of the basket are old files, that are not displayed in the basket.
To solve this issue, all files in the basket directory have to be deleted (except the basket.ini).
The most reasonable cause of this issue is anti virus software. In the configuration of the anti virus software you have to make sure, that the basket directory is excluded from the virus search. If you have not excluded the directory, it might be that right at the moment the client wants to delete the file, it is being scanned by the anti virus software. This will cause, that the file is not deleted and stays in the directory, even when it is actually deleted in the Basket.