Is it possible for Windows 11 firewall rules to cause an IDM 403 error?

Yes, Windows Defender Firewall or third-party firewalls can sometimes block IDM's access to necessary ports or resources, leading to a 403 error. Ensure that IDM is allowed through your firewall. Specifically, allow IDM's executable (idm.exe) and its associated ports (typically 80, 443, and 8080) to communicate.