As the OP did not respond, here's a community wiki answer.
The issue has been solved by using the MiKTeX Package Manager instead of automatic package maintenance, as confirmed by Ruben.
My personal experience suggests that for a MiKTeX repository "to be online" is not only a question whether that repository is accessible via HTTP or FTP, but also whether it contains valid package data.
Contary to an old comment of @ulrike-fischer written here, every time when I start the MiKTeX Update Wizzard with a fixed repository selected, the wizzard first connects to the MiKTeX Repository Web Service. My assumption is that it verifies whether the repository contains valid package data and is therefore safe to update from.
When executing mpm --list-repositories, the MiKTeX package manager queries the same service for a list of repositories that are currently containing valid package data. Certainly sometimes it marks my preferred repository as offline even when it can still be accessed via HTTP.
The current web service response to mpm --list-repositories is:
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<soap:Body>
<GetRepositories4Response xmlns="http://miktex.org/2015/06/repository">
<GetRepositories4Result />
</GetRepositories4Response>
</soap:Body>
</soap:Envelope>
Note the empty GetRepositories4Result tag: As far as the repository web service is concerned, all MiKTeX repositories are offline and cannot be used for updating. This has probably something to do with the massive MiKTeX update from May 10, 2016.
I guess it is just a question of time before repositories will get the updated version of MiKTeX and will be flagged as online.
I have always understood that that while a MiKTeX repository can be accessed via FTP or HTTP (and individual packages downloaded to your local repo), it is sometimes centrally marked as "temporarily unusable" e.g. due to an ongoing update of packages -- you certainly do not want to have your users running update from a half-updated repository, I guess.
Best Answer
I found the source! It can be downloaded, together with some other older files, from CTAN Archive