For users reporting issues with "TLS certificate verification failed" errors:
Check for and delete your expired R3 cert from LetsEncrypt.
1. Open Run and type mmc.exe
2. Select <File>, <Add/Remove Snap-In..>
3. Choose <Certificates>
4. Select <My User Account>, and click<OK>
5. Expand <Certificates - Current User>
6. Expand <Intermediate Certificate Authorities>, and Click <Certificates>
7. Find the expired R3 and delete it.
NZBGet has released an official fix specific to their newsreader here: https://github.com/nzbget/nzbget/issues/784#issuecomment-931609658
For your convenience I've prepared fixed cacert.pem: https://nzbget.net/info/cacert.pem.
Please download it using your web-browser and put it over existing file in nzbget installation:
On Windows: under C:\Program Files\NZBGet;
On Mac: /Applications/NZBGet.app/Contents/Resources/tools;
On Linux if you use installation package from nzbget download page: in nzbget installation directory, the file is near nzbget executable;
On Linux if you use Docker: inside docker container in nzbget installation directory, the file is near nzbget executable.
When downloading the file please make sure it was saved as cacert.pem, some browsers may change file extension.
After replacing cacert.pem you need to reload nzbget via Settings->System->Reload or just restart the app.
Alternative you can instead disable certificate validation via option CertCheck in Settings -> Security.