Folder list will be highlighted arbitrarily by trying drag and drop mails

Notes version 8.5.1 with FP 1Windows 7 Professional

The user can not drag and drop mail(s) to the folder in the folder list. At that time she tries to drag the mail the folder list will be highlighted arbitrarily so that she can not see to what folder she would drop the mail since all folders in the neigbourhood are highlighted.

Does anyone any idea?

Thanks