If you see this error, check that internet connection is not firewalled and you are not behind a proxy.

ppq.apple.com should be accessible, you should see "ok" or error when you are trying to access it.

If everything is fine with your internet connection, this means that Apple has banned your device from using of free sideloading with community-provided certificate.

There is nothing we or you can do with it.

You need to switch to your own private developer account purchased directly from Apple.

Please note that appdb PLUS itself does not give you ability to sign apps, it only unlocks additional features of appdb and allows you to sign more than 30 apps per month. Refund requests referring to this issue will be rejected.