AGOL New Item error for ArcGIS Server web service - "The URL you provided is incorrect"

[HenokA]

I am getting an unusual error while adding an ArcGIS Server web service URL using the "New Item" in ArcGIS Online. The services are working and visible from the Rest Service Directory while throwing this error while trying to add them as an Item to AGOL.  Is this likely caused by some server configuration settings in the ArcGIS Server hosting the services?

"The URL you provided is incorrect" is all that I get from AGOL and there is no hint as to the possible cause.  I could not find any similar error in this context on Google or in this forum.

Any lead to this issue is highly appreciated.

Thank you.

[HenokA · Accepted solution]

For anyone checking this . . 

this issue was resolved after checking DevTools that threw - CORS policy: No 'Access-Control-Allow-Origin'. 

And this ESRI documentation was used to address the issue. And this forum posting was quite useful in leading towards the solution.