Options Pages are only showing after Disabling and Reenabling the vsix extension

I'm the author of a small os vs2015 extension (AsmDude) and my users complain that the options pages only become visible after disabling the extension, restarting vs, enabling the extension and restarting vs again.

1] I have sifted through many tutorials, examples and manuals on how to use the ProvideOptionPage attribute properly, but nothing helped.

2] I have run devenv.exe with /LOG, but nothing strange comes up.

3] I have cleaned all registry entries that this plugin makes, but no fix.

4] The problem exists on clean installs of visual studio 2015 (sp2)

Next are some code snippets:

[PackageRegistration(UseManagedResourcesOnly = true)]
[InstalledProductRegistration("AsmDude", Vsix.Description, Vsix.Version)]
[ProvideAutoLoad(UIContextGuids.NoSolution)]
[ProvideMenuResource("Menus.ctmenu", 1)]
[Guid(Guids.GuidPackage_str)]
[ComVisible(true)]

[ProvideOptionPage(typeof(OptionsPageSyntaxHighlighting), "AsmDude", "Syntax Highlighting", 0, 0, true)]
public sealed class AsmDudePackage : Package {
//snip
}
[Export(typeof(DialogPage))]
[Guid(Guids.GuidOptionsPageSyntaxHighlighting)]
public class OptionsPageSyntaxHighlighting : DialogPage { 
// snip
}

Question:  how to debug such a vague problem? There must be a warning or error message logged somewhere that could help me pinpoint the problem.

Edit: this question can also be found here on StackOverflow.

Regards!