1. If Safari doesn't open a webpage or isn’t working as expected
Try suggestions given in this article.
https://support.apple.com/en-us/HT204098
2. Quit Safari, launch Safari holding the Shift key down.
https://support.apple.com/en-us/HT203987
3. Startup in Safe Mode. https://support.apple.com/guide/mac-help/start-up-your-mac-in-safe-mode-mh21245/mac.
Quote:
It also does a basic check of your startup disk, similar to using First Aid in Disk Utility.
And it deletes some system caches, including font caches and
the kernel cache, which are automatically created again as needed.
Login to your account and test the site.
If the site loads without any issue, third party software like VPN or anti-virus software may be the issue.
Logout and "Restart" your Mac.
4. Sometimes incorrect time and date can cause this kind of a problem
Set it to “Automatically” . https://support.apple.com/guide/mac-help/set-the-date-and-time-mchlp2996/mac