Your screenshot is of a folder poiting to this product and it's templates:
http://www.visionrez.com/Software/index.html
Or is that not your screenshot?
(c:\ProgramData\Autodesk...
is not the same as
(E:\C\ProgramData...
Is your add-on installed on an E: drive?
OPTIONS command>FILES Tab>TEMPLATE SETTING>SHEET SET TEMPLATE FILE LOCATION entry, plus the DEFAULT TEMPLATE FOR SHEET CREATION entry, which of the above paths is it? And which of the two locations does this entry need to be?
The whole point with add-ons is... you need to contact THEIR support for fixes to THEIR entries, to ensure you do not mess-up their setup and your usage of their product. Any advice you receive from a non-user of that add-on (me) may cripple your precious add-on and it's features. Rmember, to date you've had 40+ other folks read your posting and not touch it for a reason, fear of messing your setup up most likely.
Sorry about your frustration, but I thought I'd let you know that I'm having the same issue today. It persists in AutoCAD 2013 as well. You can make LITERALLY hundreds of changes in the Project Navigator and then hit this roadblock, which you cannot fix in Project Navigator.
My issue is exactly the same. AutoCAD adds "\My Documents" into the filepath every time and then tells me that the file does not exist.
I've posted this before on forums with no response. Just thought I'd let you know, even 5 years later, that I feel your pain.