If a REST API call resulted in an error, the response status code would indicate a general state of the problem. The following response status codes are most commonly used in ECE REST API to indicate an error:

400 - Bad Request
401 - Unauthorized
403 - Forbidden
404 - Not Found
412 - Precondition Failed
500 - Internal Server Error

For more information on the above status codes visit HTTP Status Code Definitions page of RFC2616 protocol.

Additionally, in case of an error the response body will contain a 'message' or 'developerMessage' element describing the nature of the problem. E.g.

For Knowledge APIs for Authoring:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Error xmlns="http://bindings.egain.com/ws/model/v12/gen/platform">
   <message>User could not be authenticated. Invalid session ID.</message>
</Error>

For Interaction APIs:

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<error xmlns="http://bindings.egain.com/ws/model/v12/gen/common">
   <code>400-124</code>
   <developerMessage>$order query parameter must be accompanied by $sort.</developerMessage>
</error>

Moreover, stack dumps and error messages are logged in the following files. Examining these files should provide more insight into the cause of the problem.

logs/eg_log_{HOST}_ApplicationServer.log (most errors)
logs/egpl_root_{HOST}.log (serious configuration errors)