According to Joerg Sinemus blog article:

We have recently uncovered an issue with this Service Pack 1 package that may prevent customers who have Service Pack 1 from deploying future public or cumulative updates. As a precautionary measure, we have deactivated the download page until a new package is published.

As soon we have more information, we will update you.

Apparently if you used the SharePoint 2013 with SP1 media to install SP2013SP1 you should not have the issue.  You will have the problem only if you installed SharePoint 2013 SP1 on an existing SharePoint 2013 installation.

Also see Todd Klindt’s article:

Article en français: