An I/O error usually means that a setting on your machine or local network is blocking access to our converters. If you need to upload the proof immediately, start by trying a different browser to see if that resolves the issue.
Ultimately, you may need to consult with your IT team to see if your company is using a proxy or an anti-virus software. Once that's been confirmed, send a request to have these two domains whitelisted by your IT department:
This should solve the issue permanently, but if not, please reach out to support.