.NET exception caught is unexpectedly null

Just ran into this same problem. I finally found out that I catched different exceptions with the same name, like you did:

catch (ReflectionTypeLoadException ex)
{
    // ... 
}
catch (Exception ex)
{
    // ex is not null!
    // ...
}

Both are named ‘ex’. Changing one of both names solved this problem for me, like:

catch (ReflectionTypeLoadException reflectionEx)
{
    // ... 
}
catch (Exception ex)
{
    // ex is null - that should not be possible!
    // ...
}