And I am not trying to assign blame -- I am merely unable to see why the template as posted would have that misplaced "-" symbol.
Code:
{author_sort}/{series}/{series_index:|| - }{title}
The "-" is in the suffix for the series_index field, which means it will only be inserted in the filename if there is a series_index.
I therefore concluded that the only logical explanation is that there is a typo in the new computer's configuration, and the "-" ended up outside the field suffix.
Now, if you were sending to a device I might think it is possible the device driver is specifying an incorrect override filename save template. (Since it can be set in Preferences or in the device driver, and the device driver will always win.)
But Save-to-Disk doesn't get a device driver to override it with...