DNS issues are solved now
I've found a rather embarrassing problem with our DNS, which I've just fixed.
*.gnulinuxmatters.org subdomains are still hosted at our old shared host on DreamHost, but its DNS is hosted elsewhere. And DreamHost loves moving sites from one server to another, from time to time, but these lazy folks just bother to update the relevant DNS settings on their system, without even making sure that the DNS really is hosted with them.
So when it happens, the only way I can realize it happened is when a subdomain breaks(!).
We have monitoring services set up for situations like this, which notify me when something like this happens. But this time, DreamHost also changed the IP for the subdomain Kmail uses to access my email, so I didn't receive such notifications -- Kmail used to show a message like this "Connection failed, the server said: Temporary problem, please try again later", which didn't seem that critical and because of my lack of spare time last week, I didn't check.
To sum up, the subdomains that had been down include:
I've asked DH to let us know whenever this happens again. And anyway, I'll pay more attention to the error above if I ever see it again.
Sorry for any inconvenience,
PS: For the record, getgnulinux.org and translations, as well as other sites like linuxpreloaded.com, are all fully hosted by us. So this issue will never affect them.