We are aware of an issue where -604 errors can appear when users have previously downloaded a product from Add-ons/Adobe Exchange that has now been retracted by the producer.
This issue should be resolved in the next 24 hours - please let us know if problems persist for longer than that.
Thanks for your patience,
Just an update - unfortunately this problem still persists for some users. We are working on it, and we hope to have a resolution soon.