safari refuses to upload photos

One test the Apple tech had me run was putting Safari into a "Private Window". The problem went away in that mode.

According to If Safari on Mac doesn't open a webpage or isn't working as expected - Apple Support,

If that works, use the following steps to remove the website's data, including its caches and cookies. The website can then create new data as needed. If it's a website that you sign in to, make sure that you know your sign-in information before continuing.

1. Choose Safari > Settings (or Preferences), then click Privacy.
2. Click Manage Website Data.
3. Select the affected website from the list shown.
4. Click Remove.
5. Click Done.
6. Open the website again in a non-private browser window.

In Step 3, you can simply choose to Remove All. The only effect it will have will be the need to "sign in" again to sites that require it (such as this one). If those websites have an option to "remember" you so that you don't have to sign in all the time, you will have to check that box also.

Removing all saved cookies and website data isn't a bad idea to do from time to time anyway. Most websites are far from perfect and they can misbehave if that data becomes old or outdated when changes are made to those websites. Perhaps that is what's happening in this case.

I understand. Posting that screenshot with the "Image insertion" button ruled out a number of potential causes.

If Safari on Mac doesn't open a webpage or isn't working as expected - Apple Support is frustratingly broad, but it has to be since there are so many potential reasons. However, the "repeatedly occurred" message is almost always the result of poorly written webpage code, and that flawed code is almost always due one or more page elements designed to deliver interest-based, targeted advertisements in one form or another.

Apple Support Communities may be the only website in the known universe that is not plagued with such things, which is the reason using the "Image insertion" button is instructive. That also means it's not strictly a Safari problem, although using a different browser (such as Brave, or Firefox) may also be instructive. It also means buying a new Mac won't help either. I mean it will help Apple, but it won't help you.

If Safari on Mac doesn't open a webpage or isn't working as expected - Apple Support suggests a number of potential solutions. Try reloading the page without content blockers, assuming you are using any, but if you are not using content blockers you might find they are capable of mitigating the effects of intrusive advertisements or similar content that could be the cause of poor performance.

Also, consider restarting your Mac in "Safe Mode" and determining if the same problems still exist while in that mode: How to use safe mode on your Mac - Apple Support. Your observations might also advance troubleshooting. Then, restart your Mac normally (not "Safe Mode") and observe again.

Let me know what you determine and I may have additional suggestions based on your observations.

You're welcome Chazz. It's odd that reinstalling macOS would have been required but it fixes things on rare occasions.

Thank you for your kind words.

It's harmless enough to try: Reinstall macOS - Apple Support

What happens when you try? Use the "Image insertion" tool (pictured below) as an example: