Hopefully this will help someone else since I've seen it happen a few times.
Using ArcGIS Server 10.0 and Desktop 10.0. Using a mapServer type service on ArcGIS Online, features cannot be identified on a certain layer within the service. We know that ArcGIS Online sends a URL to our local server as part of the identify operation. Going into Chrome's developer tools, then copying and pasting the identify operation URL into a new browser session results in an error 400, with no helpful details.
The mapServer service comes from our local instance of ArcGIS Server. As a test, I added the mapServer layer to ArcMap and tried to identify a feature. Identify operations work in ArcMap. But not in ArcGIS Online - strange.
In an attempt to fix this, i restarted the service, removed and re-added the layer in the ArcGIS Online map. Still, cannot identify features. Same error 400 is returned.
Using the same layers, I rebuilt the MXD, created a new service, then added it to the ArcGIS Online map. Now identify operations are working just fine. Since the rebuilt MXD solved the issue, I am guessing there had to be corruption in the MXD.
This same series of events has played out several times. At least we know how to correct the problem.
Paul Lohr
Lohr Geospatial Services, Inc.
