Problems with XML-RPC?

I’m running WP 2.1.3 (but I also tried the same on 2.0.11 and 1.5.3) in bare configuration (practically no plugins activated and no themes added) on PHP 4.4.x and MySQL 4.0.x. Posting works without any problems from inside the admin panel but any time I try to post with a desktop blogging software (I tried Windows Live Writer – latest beta) and BlogJet (latest demo) I receive error messages.

In WLW it is “Invalid Server Response – The response to the blogger.getUsersBlog method received from the weblog server was invalid:
Invalid response document returned from XmlRpc server.”

In BJ it is “the xml-rpc server returned a empty response (EGetRecentError)”.

Is there something on the server side that needs to be fixed?

Any ideas what might be causing it and how to fix it?

Thanks in advance.