I installed SP1 a few weeks ago, and generated the pfx file today, so I don't think that is causing it, but I'm going to test from a machine without SP1 installed.

UPDATE: Just tried it with a machine without SP1, same problem.
I can easily select the certificate and get it loaded, but when I build, it asks for the password again, and this time I get the messagebox and the error.

Any ideas, or any way to get a bit more usefull information that just "Object already exists", since that doesn't really point me to whats causing the problem.