Suddenly no access to adult or graphic content websites

Hmmm... that would indicate the server is refusing the connection, not your Mac.

Are you using a VPN?

Do you have iCloud private relay turned on?

If the answer to either is yes, turn it off. See if that makes a difference.

sinepgnol wrote:

However, I found out that this problem only occurs when I'm connected to my work VPN through which, however, I am not routing all my traffic and I only use this connection to connect to a remote shared hard drive. I specify this in my routing tables. When I connect to the VPN, I still have the same IP address than I had before connecting to the VPN, which confirms that I don't route all my traffic therethrough.

The VPN setup is managed by your employer. You may think you are bypassing it, but they are controlling the traffic, despite your routing tables. My employer used to have a split VPN and non-work internet traffic was supposed to be routed outside the work system but in fact that was not true all the time. Not only may they be rejecting your attempt to access those sites, they are probably logging it as well. Most employers are pretty unforgiving of this type of activity. But what is happening could even be more complicated than that: some web sites will try to find a way to detect and reject attempts to connect to them via VPN for all kinds of reasons. Suggest that you separate your personal browsing from your work computer and network.

sinepgnol wrote:

However, I found out that this problem only occurs when I'm connected to my work VPN

But didn't you say earlier:

I'm not using VPN

You are describing a complicated, non-standard configuration. Furthermore, corporate VPNs are very specific. You are unlikely to find anyone here who is going to be able to help. I recommend you contact your work IT support and I'm sure they will be able to resolve the situation.

Download and run this app so we can find more information about your installation.

https://etrecheck.com/upgrade

It shows what is launched and other information and is very useful in finding causes of problems. After you first launch the app make sure you check Enable full disk access in the box in lower left.

After running the app use the app's feature to paste to clipboard. Then paste it in the text box for a reply in this forum click on the addition text icon. This is because the normal reply text box limits how much you can type.

o personal identifiable information is contained if the app output. The app was written for a person who is contributor to these Apple Support Communities

Just to add, I would test while in Safe Mode or with a new user account.

How to use safe mode on your Mac - Apple Support

If it works when you use a VPN but not when you don't, that would indicate the host site is refusing traffic from the public IP address you're getting from your ISP.

Unless you have a business account, you should be getting a dynamic address. Rebooting your router might result in getting a new public IP address. Try that. If that doesn't work, you will probably have to take the issue up with your ISP. For some reason, the hosts are not accepting traffic from them, or at least from that network segment.

Turning on iCloud private relay might get you around the issue as well, since all web traffic from Safari is redirected through multiple proxies, masking your actual IP address.

Change Content & Privacy preferences in Screen Time on Mac - Apple Support (CA)

Have you tried a different browser?

Do you have any kind of "security" software or ad blocker? By default, any of those will block this kind of content due to the risk of malware.

sinepgnol wrote:

But it does WORK from my iPhone, which is connected to the same router, that is, it has the same public IP address.

Does it work on your iPhone with Cellular Data disabled?

Hmmm... I'm running out of ideas as well.

The connection is either being rejected at the host, or on your router.

Check your router configuration. Reboot it if you haven't done so.

It's possible there is a filter on the router itself...

Is this your own personal computer or a work computer? Who configured it? Not sure what you mean when you say "when I disconnect the ethernet and WiFi" but if you are truly totally disconnected from the internet and you get that message, then it is originating from within your Mac and how it is configured. This type of error message can result from incorrect firewall, system or browser settings, or from malware. Or from something that is cached.

Does the Mac have any security or anti-virus software?