I have the same problem, not sure the actual cause but when I select APN to digiwap then exit and goto sms/mms, click the download button and everything works. now i have reset APN to diginet. I will wait for next mms to see if I need to switch back to digiwap to download. Meantime you have to manually switch back & forth from digiwap (download mms) and back to diginet for internet access.
At least it works with a bit of trouble :-)