VERIFIED SOLUTION i
'It's not you, it's us. We're having an internal server issue.' is being displayed while attempt to sign-in into EngageOne interactive client.
One of the possible reasons is the 'core.services.url' mentioned in deploy.properties having EngageOne server's hostname rather than EngageOne server IP and user is trying to access the EngageOne interactive client using EngageOne server IP URL or vice versa.
ResolutionUPDATED: September 7, 2018
It is recommended to use the same URL as mentioned in the deploy.properties file.