Jon,
In our case, when AGS was installed, we set F:\arcgisserver as the default arcgisserver folder. (i.e. F:\arcgisserver\arcgisoutput) Whenever an upgrade was done, it reset this back to C:\arcgisserver which would not be recognized by the application when looking for the attachments in the F:\ folder. Once I set it back to F:\ the error went away. (You must make sure that the F:\ folder has the correct permissions, etc.)
So, I think the issue in our case was specific to our custom installation of AGS.
Jim