Different websites not loading. Some inaccessible. Where to look for first?

Hi all,

I just noticed that, on my daily driver Mac, a wide variety of websites were completely inaccessible, and I am not sure what to look for in search engines.

Here are the symptoms:
In all browsers, all websites load extremely slowly. Some of them don't load at all. Reproducibility: always.
Sites that don't load aren't ping-able either, as I get a "No route to host" or "host is down" error for 5 pings, then address resolves to an IP that doesn't match the real one, and I still get a timeout.
Nothing appears in the Console.

Tested:
de-activated all extensions: Glimmer Blocker, Safari extensions. No result.
Purged caches. No result.
Logged in as another user: same result.
Restarted: no result.

It is to be noted that the second MacBook I'm posting this topic from has no issue whatsoever, and both are connected on the same router. On both, DNS address is 192.168.1.1, the router's address, but only one is exposing issues. Hence, this seems to be limited to the other machine itself.

I am not sure what I should look for in search engines. What can be the issue there?

Did. Sites still load as usual on the present MacBook, but the other one still can't access it, among them Macrumors.

Tested with another user account after flushing out DNS caches with dscachetutil -flushcache. I am not being redirected anywhere. The sites just don't load. What's interesting is, even as I enter the correct IP directly to access a website, it still doesn't load. I thought this may "skip" the DNS server.

What next?

It seems to have did the trick. I still don't understand what was e issue there, but is solved so far. Lets do some tests...