Excel Error 406 when using both VBA and ActiveX addins.

I don't have an answer, but here are a couple of things you can try.

You can tell what type of Addin you are dealing with by executing File | Options and selecting the Addins Tab.

If the event happens as soon as you load Addin B, it probably means you are calling a non-modal dialog box as it states, but there are a few other things that could give you similar errors.

Based on your description, it sounds like the error could either be a dialog box in your Addin, or it could be in the other addin, and it gets called as a side effect of some state change your addin made.

To get to the bottom of it, you need to attach a debugger. You can do that by making Excel your Startup Project, or by attaching later. The former is probably easier in this case.

1. In Visual Studio, Use Project | Properties | Debug, select Start external program and put in the fully qualified pathname to Excel.

2. Load Addin B manually to give yourself the error

3. Break into the debugger and examine the call stack.

That will frequently but not always give you good clue as to where the problem is, but it's always the first step. If it doesn't give you useful information (stack info is frequently completely lost in transitions between addins), you may want to put some breakpoints in your projects in any events you handle. Even this doesn't reliably work all the time, but it's worth a shot.

If I had to guess, I would say probably have some event handlers in your add-in that are causing the problem, And you might have to do something like change a .Show to a `.ShowDialog', or defer the processing of the form until outside the event handler but that's just a guess.