Identity Server error

Identity Server was returning Error: "SSL certificate chain error: A certificate chain processed, but terminated in a root certificate which is not trusted by the trust provider"

This error is returned by REST if it doesn't trust Identity Server certificate. 
To verify this: 
- Open Identity Server administration page and find which certificate is using (It will usually be "CN = Geocortex, CN = Signing, CN = AMARI-PC2) 
- Open C:\Program Files (x86)\Latitude Geographics\Geocortex Essentials\Default\Post Install\Certificates.msc 
- Select the certificate the IS is using and open it 
- On the details tab, find the "Thumbprint" and copy it. 
- Edit the security.xml "C:\Program Files (x86)\Latitude Geographics\Geocortex Essentials\Default\REST Elements\Sites\security.xml" 
- Within the file, find the <ServiceThumbprint>EE929742F12ADE54DC07968A4ED339DA1B225AC2</ServiceThumbprint>. This value should match the certificate's one. If it doesn't change the value in security.xml (alternatively, it might be that the Identity Server certificate was changed to some other than signing and you can just change it back in IS admin page).