Well there must be some kind of information [logs] on the webdav client if uploads are not working for certain paths. Please check if there's a difference for the path where upload works and doesn't. It might be worth to try a different webdav client
Another way you could add the resources is through code, which is suitable if its a one time job and build the application package as part of maven project
is it only path in /etc/fmdita/dita_resources? Does it work for other paths?
Checked drag and drop from local drive to mount drive on MAC for a different path on my 6.4 instance and was able to add multiple files to AEM through webdav