This error is displayed when archiving a long string of nested forwarded messages. It is a SOAP error and doesn't impact Retain's ability to archive the message.
To find the subject of the message receiving the error. Open a RetainWorker log from a day where an archive job was run and you received this error. Search for: "GWPOA HTTP server returned http error (gw error): 500(0x8929) , Code: 8929", then search up for: "subject:"
If you search the message in GroupWise webaccess and open the attached forwarded messages, you will eventually get this screen:
The error is a SOAP error that is being returned to Retain in a request for data. In Retain, this error can be ignored.
If it has prevented the advancement of the retention flag, you can manually advance the flag using the Retention Flag utility without worrying about data loss.