Can't run my own plugin "Could not read add-in file: ..."

Every time I try run a plugin, which i compiled myself, I get the following error:

ERROR: Could not read add-in file: /home/marf/.local/share/gnome-do/plugins/<plugin_name>.dll

Compilation didn't throw any warnings or errors. DLL is readable and obviously deletable by Gnome-Do ;)

OS is (X)Ubuntu 8.04
gmcs version is 1.2.6.0
MonoDevelop version is 1.0
Gnome-Do version is 0.5.0.1

What I did:
* followed the tutorial 'BuildingPlugins', compiled via gmcs -> fail
* used MonoDevelop to compile 0.4 plugins from the source package -> fail
* compiled Gnome-Do from the source package, built plugin -> fail

Where it fails:
I pinned it down to ...
PluginManager.cs
  internal static void InstallLocalPlugins (SetupService setup) { ...
    setup.BuildPackage (...

What I tried:
* modified the PluginManager in hope to get more information about why it doesn't load the plugin
* wrote a simple statusMonitor (IProgressStatus) to hopefully catch an Exception, but didn't get any more information besides the above mentioned Error

What I noticed:
* the plugins in the tarballs use a Do.Addins package version 1.0.+++ (according to MonoDevelop Projects)
* to build my own plugin I had to reference the registered package Do.Addins version 0.5.0.1

Any pointers where to look next?

Thank you for Gnome-Do ... I love it ... Now if i only could run my own plugins :)