I\'ve just pulled down the latest Swagger from the Git repo (3.0.19) using: https://github.com/swagger-api/swagger-ui.git and updated my API to use the new version.
This error is generic on swagger side and could be due to many possible reasons.
In my case, it was due to connection error. My swagger page was not responsive due to connection issue at my side. I had to refresh it once and worked for me.