Do you have CAS array configured?
Please try to use https://www.testexchangeconnectivity.com/Default.aspx to test the connectivity.
For the warning, I recommend you to have a ExBPA to have a health scan on the CAS/HUB server and then report the error here.
Besides, I' d like to know if any other error event logged in the application log. Please try to post the detail error event information here for futher troubleshootings.