This error seems to stem from the “Update” dashboard widget. Maybe (temporarily) delete this widget from the dashboard (manager/?a=system/dashboards).
You can ignore this.
Try setting the system setting auto_check_pkg_updates to “No”.
There seems to be a problem with getting the update information from the package provider. The changed system setting value won’t fix it, but you should at least see the list.
I’ve seen similar problems before, but I still don’t know why some people have problems requesting package information with cURL from the package provider.