This message means that TLS is unable to validate the host certificate against the Root CA (Certificate Authority) certificate.
If the certificate was generated by a major Certificate Authority, the Root CA certificate should be in the Windows certificate store. Please check the TN3270 Plus "Use Windows certificate store" option (Host, Connect..., Advanced... button, Setup Items = Security, Use Windows certificate store check box.)
If the "Use Windows certificate store" option is already checked, set the "Display certificate when connected" option. (Host, Connect..., Advanced... button, Setup Items = Security, Display certificate when connected check box). Capture a screen image of the certificate and email the screen image firstname.lastname@example.org
TN3270 Plus is continually being improved. To learn more about the latest enhancements, please review our Version History web page.