You might have landed here searching for the meaning of “not a valid Win32 FileTime” while trying to run a Duplicati backup. The solution is to find files with an invalid access time (atime). In the simple case where Duplicati emits an error for a particular local file path, you can edit the attributes of the file in question on your machine. I wrote this post in case like me, the Duplicati log does not give the location of the invalid file — because it’s in the backend.
I diagnosed this by using the “Live” log, starting the broken backup again and observing the error is thrown after the Backend event: List log entry. I’m using samba to backup parts of my Windows machine to a server in my house using the samba protocol. From the server I listed the target backup directory with ls -lutr (list, using access time, sort and reverse); and immediately noticed several blocks from the last successful backup had an access time from the year 30828!
I touched the affected files to give them a more sensible access time and the backup from my Windows machine was able to complete successfully. If I ever discover how this happened in the first place, I will update this post. In the meantime, I hope this is helpful to someone else as searching for help on the issue only raised old problems with local file backups.