The default self-signed certificate has expired on the on-premises appliance.
Agile Central On-premises 2016.1
Agile Central On-premises 2016.2
Agile Central On-premises 2017.1
Agile Central On-premises 2018.1
The certificate that ships with the on-premises is intended to serve as a temporary placeholder certificate. This certificate is not trusted by any browsers as it has not been signed by a trusted certificate authority. It should be replaced by a certificate that is trusted by your organization or provided by a third party certificate authority.
Please see the following article for steps to generate a signing request:
We understand, however, that due to financial constraints or internal resources it is not always possible to acquire a trusted certificate. Because of this, you can find a new certificate attached to this case. Please download that and follow the instructions here: