Problem with Campaign.Create

I'm getting the following erorr when I try to create a campaign through the API:
   
Campaign.Create Web Service method name is not valid.

I'm using the same method as for all other functionalities in the api, they all work, but I cannot get this one to work. I also had a look at the PHP5-class, but using that is no option (PHP4 user :-( ), but when I compare the code, I'm using the same variables. Are there limitations or am I doing something wrong?

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