Based on the error message (Patchstack Error Code: 21871), it appears that the download request is being blocked by the server’s security protection rather than by WordPress Download Manager itself.
Please contact your hosting provider and share the screenshot with them. Ask them to check their Patchstack security logs and investigate why the password protected download request is being blocked.
If necessary, they can coordinate with Patchstack support to identify the rule triggering the block and whitelist the legitimate download request.
Once they have investigated this, please let us know their findings.