You’ve previously tried to address this issue from another user within this post:
sourceforge.net/p/xltoolbox/disc … /0fe4f8a6/
I’ve encountered the same issue but with Excel 2010 in Windows 7. I understand according to your post that you do not have Windows 7 to test this, but I believe I’ve came across an interesting solution:
social.technet.microsoft.com/For … excel-2010
Is the SetDllDirectory call used in your addin?
Thanks for your hard work with this.