Updated September 29th, 2020.
The problem: you’re using Safari, and you’ve updated Safari to version 14, and websites give you messages about “this page was reloaded because of a problem” and the page never shows.
I had this problem myself.
Here’s the message I got when I tried to load eBay.com using Safari 14 on my MacBook Pro with Mojave (10.14.6) installed:
I tried quitting Safari and relaunching (no good).
I tried turning off extensions in Safari’s Preferences (no good).
I tried restarting the Mac (no good).
I really thought I was stuck. But then I remembered: it’s easy enough to reinstall macOS, and it doesn’t delete your files or anything like that, so why not try? I tried, and after the reinstallation of macOS, Safari 14 worked.
How to reinstall the system
Restart your Mac, and immediately hold down Command-R. (Your Mac should be plugged into the wall– don’t try this running on battery power.)
Your Mac will boot more slowly than usual, but eventually it will finish, not at the Desktop but at something called macOS Utilities.
Double-click the “Reinstall macOS” option. Then tell the installer that you want to install macOS onto your startup drive. You will probably only see the one drive but you do have to click on it to continue.
I know this is scary but the way this works is it leaves your personal stuff (documents, pictures, downloads, music) alone, while replacing the system. When the process is complete your Mac will still look like your Mac. All of your files will still be there, your desktop picture will be there, your Dock will look like your Dock, and you’ll still sign in as usual.
And, if your Mac is like my Mac, Safari 14 will work just fine.
Interestingly, reinstalling macOS solved a problem connected to the Parallels Desktop 16.0.1 update. After the update, Parallels would no longer start, but after reinstalling the system everything was fine again.
Naturally having a backup of your Mac would be a good idea before reinstalling macOS, just in case. See my article about Time Machine backups for more information.