This is usually because the file being uploaded exceeds the maximum limit of 1mb. Please make it smaller, as a text file, it shouldn't be bigger,

Also, please check to see if you have any extensions, adblockers, VPN's, proxies or anything else that would interfere with your usual browsing experience. If it is, try turning it off.
Alternatively, please use another fresh web browser or try a private (incognito) window, as this usually disables these things on the web browser side at least.

If you're still having issues, please contact support.