Could you share more information (I am assuming your ArcGIS Enterprise is deployed on Windows Server environment (IIS)?
1. Version of ArcGIS Server/Enterprise? Is it federated with Portal?Version of OS?
2. Was the link working previously, or is it a fresh install? Did the issue start after any particular activity (e.g. Upgrade, or Changing http protocol in Server Admin)?
3. Check if the ArcGIS Server service (Run > services.msc) is running.
4. What type of SSL certificate are you using? Is the SSL certificate valid? Are you able to access " https://gisserver.domain.com" ?
5. Are you able to access REST directory or Server Admin page?
6. Are you able to access " http://gisserver.domain.com:6080/arcgis/manager"
*Meanwhile log a case with Esri support team as well.
Think Location