Error 429 is 'ActiveX Component Can't Create Object'
Your Excel code cannot create an instance of the COM component - Did you definitely create the class library as a COM object? You cannot just access a .Net DLL from COM without a bit of plumbing to make it accessible.
Building COM Objects in C#[
^]
You should then have a
ProgID[
^] for your DLL, which you can use to create instances.