Why is my Mac refusing to open certain websites?

Hi,

I’m having a weird problem where my MacBook Pro is refusing to connect to certain mainstream websites, which have always worked fine in the past.

I first noticed this in Safari, but I've checked and the problems also occur in Firefox and Chrome too, reporting error messages as follows: 

Safari: Safari can’t open the page “[URL]” because Safari can’t establish a secure connection to the server “[domain]”.

Firefox: Secure Connection Failed. An error occurred during a connection to www.instagram.com. PR_END_OF_FILE_ERROR

Chrome: ERR_SSL_PROTOCOL_ERROR

This happens with a range of websites, including https://www.instagram.com and others, all of which are modern, mainstream websites so it isn't that they are insecure websites or anything.

Other devices connected to my network have no problem reaching the same URLs.

Interestingly, if I turn on my VPN service on my MacBook Pro (I use NordVPN) then the sites work perfectly via the VPN, but not when it is turned off.

In trying to identify the cause, I should mention having used the app SelfControl to block the BBC News website, that block has now expired, but the problem started occurring within a couple of days after that; however, I’ve done everything related to the app that I can (including reinstalling, following all guidelines, and subsequently deleting) but the problem persists, so I’m not sure if that has anything to do with the problem or not.

I normally run Malwarebytes but I've tested with this installed, and uninstalled, and that makes no difference - the problem persists.

Anybody got any bright ideas? Is there some hosts file which may have got corrupted which I can flush? I'm at a loss as to how to proceed.

Thanks.