ClientState.selectedMessages does not work

Hello, we have been trying to use a C3PO to get the Message ID of a selected Mail. (We use C# and GroupWise 8)

The code we used is claimed to be working fine.

C3POTypeLibrary.IGWClientState6 cs = (C3POTypeLibrary.IGWClientState6)ContextStuff.g_C3POManager.ClientState; 
object o = cs.SelectedMessages;
MessageList ml = (MessageList) o;

Building the DLL works, registering works fine as well.

However, our C3PO is a RightClick Context Menu, so we want to get the messageID of the Message after right clicking it and then clicking our C3PO Context Menu Command.

After clicking our Context Menu, we get an very vague Error Message saying "Error exceuting a C3PO Command. Ask the System Administrator"

Is there any way to debug a C3PO, since the mentioned error is of no use?

And i am very confused, because the code we used is widely spread over the net and confirmed as working.

The Error does not occur if we skip the last line of code (MessageList ml = (MessageList)o;)

Also there are three methods working normally for the clientstate object: The method for clientname, major and minor these methods do not return an object but a "real" type like string and int.

Thanks for any help.