User can't open files in win-explorer using WebDAV

a user is member of the group “HR” and the folder “Administration” is shared with “HR”. In a subsubfolder another user uploaded some files(ppt, pdf). Now the first user would like to edit the files but cannot open it throw the winexplorer; If he uses the webinterface, he can download and edit it… Other files in the folder “Administration” making no problems.
Error when opening the ppt: the information provided cannot be downloaded

Can’t figure it out what is happening^^

Hope someone can help me :sweat:
kind regards