I don't know if this will help you or not but....
Outlook is "dumb". It cannot figure out which version of the .NET
framework it should use to load your .NET addin, and will always use
the newest version. You stated that your addin was written with VS.NET
2003 which means that it works with the .NET framework v1.1 However,
Outlook, being "dumb" is going to try to use the .NET framework 2.0 to
load your .NET 1.1 addin. This obviously will not work. You need to
tell Outlook which version of the .NET framework it should use. To do
this, make a new file named "Outlook.exe.config" and put it in the same
directory as your Outlook.exe. Inside of this file enter:
<configuration>
<startup>
<requiredRuntime version="v1.1.4322" safemode="true"/>
</startup>
</configuration>
This will tell Outlook which version of the .NET framework it should
use when trying to use a .NET addin. You can obviously change the
version value to anything valid. The reason your load behavior is
changing is likely because your addin is failing because Outlook is
trying to open it with the wrong version of the framework. Putting
this file in the proper place should solve your problem. (If you have
converted your addin to VS.NET 2005, you have opened up a whole
different can of worms. Depending on what your addin does, its NOT a
direct conversion, it takes some work to make it work properly.)
Kelly Johnson