Plesk - Troubleshoot the "Secure Connection Failed" error

If Firefox can't access a secure site (one that starts with https) you will see an error page with the heading Secure Connection Failed and a message about the error. This article describes some of these error messages.

Certificate warnings

Firefox uses certificates on secure websites to ensure that your information is being sent to the intended recipient and can't be read by eavesdroppers. Certificate will not be valid until (date) (site name) uses an invalid security certificate. The certificate is not trusted because the issuer certificate has expired. The certificate will not be valid until (date). (Error code: sec_error_expired_issuer_certificate)

This error can occur if your computer clock has the wrong date, in which case the date given in the error message will be in the past. To fix the problem, set your system clock to today's date and time (double-click the clock icon on the Windows Taskbar). The certificate expired on (date) (site name) uses an invalid security certificate. The certificate expired on (date). (Error code: sec_error_expired_certificate)

This error occurs when a website's identity certification has expired.

This error can also occur if your computer clock has the wrong date. To fix the problem, set your system clock to today's date and time (double-click the clock icon on the Windows Taskbar). The certificate is not trusted because the issuer certificate is unknown (site name) uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. (Error code: sec_error_unknown_issuer)

The file cert8.db in your profile folder may have become corrupted. Delete this file while Firefox is closed.

Open your profile folder:

Click the menu button New Fx Menu , click help Help-29 and select Troubleshooting Information. 
The Troubleshooting Information tab will open.
Under the Application Basics section, click on Show Folder. 
A window with your profile files will open.
Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.

Click the menu button New Fx Menu and then click Exit Close 29 .
Click on the file named cert8.db.
Press Delete.
Restart Firefox.
cert8.db will be recreated when you restart Firefox. This is normal.

The certificate is not trusted because no issuer chain was provided (site name) uses an invalid security certificate. The certificate is not trusted because no issuer chain was provided. (Error code: sec_error_unknown_issuer)

You may have enabled SSL scanning in your security software such as ESET or Bitdefender. Try to disable this option. The certificate is not trusted because it is self-signed (site name) uses an invalid security certificate. The certificate is not trusted because it is self-signed. (Error code: sec_error_untrusted_issuer)

or (site name) uses an invalid security certificate. The certificate is not trusted because it is self-signed. (Error code: sec_error_ca_cert_invalid)

Self-signed certificates make your data safe from eavesdroppers, but say nothing about who the recipient of the data is. This is common for intranet websites that aren't available publicly.