Hi Dev Are you using a Safari on a Mac by any chance? If so, do you have any other browsers installed that you could try that page with, just to see if that is the issue? There are more details about Safari giving this error here... https://softwaretested.com/mac/different-ways-to-fix-safari-cant-open-page-error-on-mac/
Testing from here on your site went fine yesterday using the native ECT upload/download functionality (not so the mods) in Chrome on Mac. Safari and Firefox seemed like they might be problematic but we didn't explore that further. We uploaded a max of 104 products because that's all there were in the file we used; it only took a fraction of a second and there was no reason to suspect a larger file would not also be fine.
Peter
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