The ERR_SSL_PROTOCOL_ERROR you're experiencing on some networks but not others is typically caused by Internet Service Providers (ISPs) or corporate networks that inspect TLS handshake packets and make dynamic decisions based on your domain's reputation 1.

This can happen because:

1. The domain's reputation may be calculated differently by various ISPs using different data sources and algorithms 1
2. Corporate networks may have specific security policies that affect TLS handshakes 1

To resolve this issue, you have a few options:

1. If you're on a corporate network, you should discuss the issue with your network administrator to explain why the block is incorrect 1
2. For home or cellular networks experiencing this issue, you'll need to contact your ISP directly 1
3. As a temporary workaround, you can try accessing the site from a different network or device 1

If you're still having trouble, you might want to verify that your domain's DNS configuration is correct by:

1. Checking that your domain is properly added to your Vercel project 2
2. Verifying your custom domain is correctly pointed to the provided Vercel CNAME/A records 2
3. Clearing your browser cache and flushing DNS caches on your machine/network 2