To enable logging for the API, please log into SmarterMail as an Administrator and click on Troubleshooting in the navigation pane. On the Options tab, on the Process Logging card, ensure API Service is set to Detailed. You'll then be able to review API logging on the View Logs tab by changing the dropdown to API Service.
Unfortunately, it's hard to say what could be causing this error, as there is a whole block of code that could be failing: 1.) couldn't load domains; 2.) couldn't retrieve disk space by domain; 3.) couldn't get stats per domain; or, 4.) couldn't concatenate all the data together.
The review to check whether this API call is working as expected would be best suited by our Support Department. If you are interested in contacting Support for troubleshooting, I'd be happy to reach out to you via email to gather your account details. Please let me know.