PR_END_OF_FILE_ERROR on 2 of my 3 WordPress sites but only from my MacBook Pro I get the PR_END_OF_FILE_ERROR error on two of my three WordPress sites when using current versions of Firefox and Safari on my late 2013 MacBook Pro, macOS 10.15.6. All three sites are hosted on the same ionos.com plan (shared Linux server). I can access the sites from this computer when using Edge (Chromium) and Tor browsers. I can also access them with any browser from my iPhone, iPad, and my wife's late-2013 MacBook Pro with the same macOS and browsers.
When I uninstalled Sophos Home Premium, as attrib suggested, the issue went away. Unfortunately, attrib's solution did not work for me. When I reinstalled Sophos the issue returned.
I also use a Winston Privacy appliance but it filters traffic for our whole home network and should affect all of our devices in the same ways. To be sure, I bypassed Winston and had the same issue.
I let Little Snitch run in passive mode. It did not record any blocked connections to the affected sites. I've also asked friends to visit the sites and they had no issues.
All of this seems to point to some issue on my MacBook Pro but I have been unable to find relevant guidance I can follow to get to the bottom of it.
I am not an under-the-hood expert on macOS or networks but I usually can follow explicit, step-by-step instructions.
I would use the developer console in your browsers to start to look at the web site errors. That almost always shows you the error in these cases, however if there are lots of errors, you may spend some time eliminating them one by one.
Here are step by step directions to get the console on Safari. There are some amazing tutorials on FireFox and I know a lot of people prefer that for this exercise so maybe someone has that experience to share here as an edit or another answer.

https://support.apple.com/guide/safari/use-the-developer-tools-in-the-develop-menu-sfri20948/mac
https://support.apple.com/guide/safari-developer/welcome/mac

The benefit of these tools is you will have actionable items to take to vendors or ask wordpress experts how that error works and which precise URL triggered it. Most Wordpress pages have about 100 to 300 elements on “one page” so the vast majority of things can work and you still see an error thrown.