Software Update Manager (SUM) MailNotifier execution results in this error:

ERROR SumMailNotifier (Version 2.1.1) finished with errors
com.sap.lm.sl.upg.notifier.MailNotifierException: org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Premature end of file
at com.sap.lm.sl.upg.notifier.SlpMonitorRequestFactory.build(SlpMonitorRequestFactory.java:58)
at com.sap.lm.sl.SumMailNotifier.<init>(SumMailNotifier.java:198)
at com.sap.lm.sl.SumMailNotifier.main(SumMailNotifier.java:542)
Caused by: org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Premature end of file.

OR, 

[22.04.2022 08:38:46] ERROR SumMailNotifier (Version 2.1.1) finished with errors
com.sap.lm.sl.upg.notifier.MailNotifierException: javax.net.ssl.SSLHandshakeException: java.security.cert.CertificateException: No name matching <fqdn or hostname> found

Sapstartsrv.log from /hostctrl/work/ dumps with below error:

[Thr 1456]   SSL_get_state()==0x1180 "TLS read client certificate A"
[Thr 1456] *** ERROR in SecuSSL_SessionStart: SSL_accept() lasterr 0x20001046
[Thr 1456]   => "received a fatal TLS certificate unknown alert message from the peer"
..
[Thr 1456]    => "received a fatal TLS certificate unknown alert message from the peer"
..
SapSSLSessionStart(sssl_hdl=00000211BD409890)==SSSLERR_ALERT_CERTIFICATE_UNKNOWN

• Systems running on Netweaver 7.x
• Software Update Manager (SUM)

MailNotifier, certificate, keytool,org.xml.sax.SAXParseException,SumMailNotifier.java, Premature end of file, No name matching, SlpMonitorRequestFactory.java:58, com.sap.lm.sl.upg.notifier.MailNotifierException, com.sap.lm.sl.upg.notifier.SlpMonitorRequestFactory.build, javax.net.ssl.SSLHandshakeException, java.security.cert.CertificateException, SSSLERR_ALERT_CERTIFICATE_UNKNOWN, received a fatal TLS certificate unknown alert message from the peer, ERROR in SecuSSL_SessionStart, SecuSSL_SessionStart, SSL_accept() lasterr , KBA , BC-UPG-TLS-TLA , Upgrade tools for ABAP , Problem