Could my firewall or antivirus be causing the 403 error? How can I troubleshoot this?

Yes, your firewall or antivirus software might be blocking access to DeviantArt. Try temporarily disabling your firewall or antivirus (use caution when doing this and re-enable it afterward). If the problem resolves, add DeviantArt to your firewall/antivirus's whitelist or exceptions list. Consult your firewall/antivirus documentation for instructions on how to do this. Ensure your firewall/antivirus isn't mistakenly flagging DeviantArt traffic as malicious.