The quick and dirty, and not recommended option, is to set CertCeck to NO in NZBGet.
Again: not the recommended wway, as this allows for a man in the middle attack.

The correct way is to delete the certificate of "DST Root CA X3" in the file cacert.pem.

The cacert.pem file can be found here:

QNAP: /share/CACHEDEV1_DATA/.qpkg/QNZBGet/bin
macOS: /Applications/NZBGet.app/Contents/Resources/tools
Windows: C:\Program Files\NZBGet

Linux users: check the directory where the NZBGet binary can be found (like seen with QNAP).

Open the file cacert.pem, and the certificate section for  "DST Root CA X3"

DST Root CA X3
=========
-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----

Alternatively, and I have not tested this, download a new cacert.pem file here: https://nzbget.net/info/cacert.pem