Safari error: "The operation couldn't be completed. (kcFErrorDomainCFNetwork error 303)".

Safari/Preferences/Advanced - enable the Develop menu, then go there and Empty Caches. Quit/reopen Safari and test. Then try Safari/History/Show History and delete all history items.  Quit/reopen Safari and test. You can also try try Safari/Clear History…. The down side is it clears all cookies. Doing this may cause some sites to no longer recognize your computer as one that has visited the web site. 

Try setting up another admin user account to see if the same problem continues. Please post back on whether or not this worked. Also try the Safe Mode. Please post back on whether or not this worked.

Isolating an issue by using another user account 

Safe Mode - About

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply        Pastebin

Etrecheck – System Information

The problem is a result of Safari holding too much local data for the site in question and failing to deal with that correctly. I use archive.org *a lot* so I had lots of LocalStorage and Cookie data for that site. The fact that this data is only in my main browser Safari and not in any of my other browsers explains why they could browse the site just fine.

So, the solution:

1. Go into Safari > Preferences > Privacy

2. Search for the affected domain name (for me this was "archive.org")

3. Click Remove

4. The problem goes away!

Here's a video showing the problem before and after: https://imgur.com/gallery/d1P1FCi

You are welcome. Glad to help.