I am using DW6. Users in remote sites are using Contribute 5. We've done this for years. Suddenly, when they try to upload .pdf files, they get an error of "Network or file system error." When I try to upload .pdf's in DW6, I get " skipped because the old remote file could not be deleted" We can edit .html documents and upload them but not other files. Nothing had changed. I started with one contribute user and now it is affecting all of them and it's affecting all of my DW.sites, even ones where Contribute is not enabled.
That line about not being able to delete the old file would lead me to thinking you might have lost some of your server privileges.
Talk to your hosting company and make sure you have full privileges on the server.