Unable to connect to indexer nzb.su SSL error

Sonarr version (exact version): 3.0.6.1342
Mono version (if Sonarr is not running on Windows): 5.20.1.34
OS: Synology NAS
Debug logs: https://pastebin.com/nsDw705X
Description of issue:
The indexer nzb.su is throwing an SSL error and won’t connect. Other indexers such as nzbfinder and usenet-crawler work fine.

the error seems is: (full log above in pastebin)

-- : Unable to connect to indexer, check the log for more details
2021-10-11 12:39:49.4|Error|X509CertificateValidationService|Certificate validation for https://nzb.su/api?t=caps&apikey=(removed) failed. RemoteCertificateChainErrors
2021-10-11 12:39:52.0|Error|X509CertificateValidationService|Certificate validation for https://nzb.su/api?t=caps&apikey=(removed) failed. RemoteCertificateChainErrors
2021-10-11 12:39:52.1|Warn|Newznab|Unable to connect to indexer