@GazHyde BTW. If you've recently migrated to a content delivery network (CDN) such as CloudFlare your problem might well be caching on edge servers rather than a User/OS/Browser issue.
Using a CDN stores cached images etc closer to the visitor's geographic location but might also have cached some non-functioning upload script which might cause issues to some users no matter what browser they're using. Usually sorts itself out once the edge servers refresh their cache.
Actually a CDN is a very wise move if your server is struggling with traffic. All static data such as images is cached and served by the CDN and it can also be configured to block those over-busy AI bots while allowing the friendly bots (googlebot, Bingbot etc) through.