Have you figured out the cause of this? I'm getting this same issue. I'm using VS 2003 and K2.Net 2003 (Build 3.6262). I did not have this issue initially, and was able to create one workflow (after applying the Smartform hotfix).
I have since tried to create one or two small sample apps and I got the same "bombing" you mentioned. I reinstalled the K2 Studio, and now I don't get the "cannot load/a reference cannot be...." message, but the Toolbox is still empty when I create a project.
Also...for the record, I'm not trying to reload projects, I'm creating NEW projects each time (yea, doing a lot of deleting) to force a build/link from the same K2.Net Studio DLL files.