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c Relativ paths in class library for application?

I would assume that you are trying to write to the file system from your application. In that case the "C" comes from the server. ASP.Net and IIS know where your application resides on the server and are mapping the path from the URL to the file system (C:). It works when you start things from Visual Studio because it's being run under an account that has write permissions to the file system. The IIS Application Pool User that is running on your server does not have write permissions. To resolve this issue you can give that user write permissions. You can see how to do that in this post.