500 Error when loading an external document

We have had Zoho writer integrated into our web application for several months, and it has worked smoothly. Today, the integration stopped working. I got the following error when I access

http://export.writer.zoho.com/remotedoc.im?apikey=xxxxxxxxxxxxxxxxxxxxx&output=editor

“Error occurred while processing your request. Please write to us at support@zohowriter.com”

Our customers are experiencing the same error. Is this a known issue, or unique to our situation?