Turns out this server and along with it approximately 64k IP addresses are blocked from editing Wikipedia, because apparently some customers of Hetzner – my hoster – run open proxies.
I appealed the block and will have to see what comes off it.
Update: I wish I hadn’t asked. Of course it was “reviewed” and declined. My guess, if spammers or vandals would only spread their IP ranges enough, at some point some “admin” (admins are normally there to facilitate work, not to hinder it) would actually block 127.0.0.1/0.
// Oliver