"No FormURN for this page"-Error after importing reusable workflow from SPD in VS

Since this morning I am experimenting again with the new workflows in SharePoint 2010. This time I tried to import a workflow I created with SharePoint Designer 2010 and afterwards import it in Visual Studio 2010 to extend its functionality. This MSDN Tutorial seemed to fit my requirement.

But after finishing the last step and the try to start the workflow, I got the errormessage "The workflow template has specified no FormURN for this page.". Hm, I thought, I forgot some step or misconfigured something but I did all well. Spent some time googling and finally found this blogpost.

It says, I should remove all the ...URL-Attributes from the Workflow-Tag in my Elements.xml. So I did and removed InstantiationUrl="_layouts/IniWrkflIP.aspx" and 'et Voila', the result was like the MSDN-tutorial promised.

But I still don't have any clue why the workflow designed and directly published from SPD does work but after importing and publishing from VS does not before I do the descriped modifications.

Any ideas and comments are welcome... :-)