Sure, but that's a very broad statement.

As I was pointing out, the OP's error message does not match (at least what he believes to be) his configuration. Why - we don't know. He might have tried to tweak the repository URLs since he unboxed his N900. You know like - some people do. I did

So unless we know, this is the first thing to check ("App Manager" -> "Application Catalogs").

Now if those URLs are correct, we can go on and try more esoteric explanations like: