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Scenario:

When login to KSC Web Console, it shows the following error:

Administration Server uses an untrusted self-signed certificate. Please modify the application configuration by specifying a valid certificate for Administration Server.

Alternative wording (for older KSC versions):
Administration Server has untrusted self signed certificate. Please, reconfigure the application with correct certificate for Administration Server.

Reason:

KSC certificate is set when Web Console is installing. If there are any changes/errors with the certificate after the installation, KSC Web Console will show this error, e.g. you installed Web Console with KSC together, then restore a KSC backup.

Solution and Source:

Change certificate in KSC. 

Specifying certificates for trusted Administration Servers - guide on specifying a new certificate