It is actually due to a bug in the WCF stack on Silverlight, where multiple simultaneous connections to the same server may (and does) result in random 404 errors on some of the requests.
Sometimes they work, sometimes they fail, you can't predict. Nor can you do anything about them...
So we put a (hopefully temporary) workaround into the data portal so it only allows one request at a time. This solves the issue, but obviously isn't ideal.
On the upside, in most business apps you aren't doing a lot of simultaneous requests anyway. Get a customer, save a customer, get an order, save an order.
Early in an app's lifetime there are probably a lot of NVL objects and maybe some read-only lists, being loaded and cached - so this hurts that process. But for most of an app's lifetime only one request is occurring at a time anyway.
Not that I want to keep this workaround!!!! I'm just saying it isn't the end of the world.
Rocky