The type ‘UserControl’ does not support direct content
For anyone who having this problem on Visual Studio 2015, try to add (if it’s not already added) System.Xaml reference to your project. Visual Studio simply fails to show reference error.
outlook-addin
How to troubleshoot a VSTO addin that does not load?
Here’s a bit more detail on RobertG5’s solution (too long for a comment): The problem was that the add-in had been hard disabled by Outlook. As I’ve learned, that’s something different than the “usual” not-loading scenario. The key to realizing this was to notice that the add-in did not show up under Inactive Application Add-Ins, … Read more