I was able to reproduce the same error message, and in my situation the Scheduler Host was not reachable from the denodo solution manager. As soon as the Host name was reachable and the ports 8000 , 7998 were opened, I was able to connect to the scheduler from the solution manager.
Also you can find explicit error messages through the denodo scheduler log located under <Solution_Manager_Home> \logs\scheduler\scheduler-admin.log.
I would also recommend taking a look at the [Default Ports Used by the Denodo Platform Modules](https://community.denodo.com/docs/html/browse/8.0/en/platform/installation/appendix/default_ports_used_by_the_denodo_platform_modules/default_ports_used_by_the_denodo_platform_modules#default-ports-used-by-the-denodo-platform-modules).
Hope this helps!