We received one Web Services Error – 604 yesterday. Based on some of the previous conversations, it seems this is API related. Any suggestions on how to troubleshoot this or should I just contact support?
Support will be able to tell you what triggered it, and possibly a bit more, but won't be able to help with troubleshooting any of the ways to fix it
Here is the full list of codes with descriptions: http://developers.marketo.com/rest-api/error-codes/
Since you will not have feedback into your specific error, I would log a case with Support.