Can firewall or antivirus software cause error 651, and how can I check?

Yes, overly restrictive firewall or antivirus software can sometimes block the connection. Temporarily disable your firewall and antivirus software. Then, try connecting to the internet. If the connection works after disabling them, you'll need to configure your firewall or antivirus to allow PPPoE connections. Consult your software's documentation for instructions on how to do this.