Adam,
It's .NET version 6.0, I just double-checked my bin directory.
The templatepath is a physical path to a subdirectory in my development machine, which is "c:\data\template\". I also tried hardcoding the physical path, same result.
In debug mode secset.TemplateGalleryPath value is"C:\\data\\templates\\CCC\\HTMLTemplate". This subdirectory exists. Can you reproduce this problem?