Sharepoint integration > invalid serial number

In the guidelines of K2 they say K2 and Sharepoint work as 1...
Well... that's not exactely true.
I've tried to make a webform(ascx) and tried to plug it in with a Sharepoint SmartForm. After recompiling (http://forum.k2workflow.com/viewtopic.php?p=1345&sid=1465e374bdd0be7d1e967e6ad99b5209 ) a lot of DLL's It finally worked. BUT due to the recompiling I've lost the possibility to select my dataFields from a K2 workflow(for some reason.... dunno why).
To be more precisely, I wasn't able to choose in the list(VS2003) to the Sourcecode.* in the properties-pane.

Anyway,
after re-installing the original DLL's I've started all over with regular aspx-files (exactly the same; no copy paste) in a new Solution(VS2003), new project with the existing K2-Workflow.
Problem?
When I've finished debugging, I received the simple message:
Server error in /project2 Application
Invalid serial number
....blablah

Can't find anywahere on the web what I should do now