I have been dealing with this exact situation ALL DAY. Unfortunately, google did not provide much help as it seems to be a new problem. Background information: I just bought a new HTC One M8 (T-Mobile) and received an MMS from my boyfriend within minutes of turning it on (no issue). 1 day later, I installed the "System update" and when he tried to send me an MMS, I got the same error as the thumbnail above and was unable to download. The error message "Message not downloaded - Generic Network Issue" kept coming up. I tried googling and doing my own troubleshooting, I restarted the phone multiple times, I messed with the APN, I took out and reinstalled the simcard, all to no avail. I contacted HTC via their live chat and waited in a queue of 40 people They also tried to get me to update my APN information and again, to no success. I then called T-Mobile to see if there was a network issue with the SIM card. By this point, I got a MMS from another friend which also resulted in the same error message. T-Mobile tech was super helpful, tried sending me pic messages which actually came through! I then realized, the two users who sent me pics were both iPhone users. So the T-Mobile tech grabbed an iPhone and tried sending an MMS at first to no avail. He tried again and BOOM, picture message received. Still, he couldn't understand why it was happening still when my boyfriend and friend were sending me pics. Hours later, and still befuddled (tmobile tech said he would call later to follow-up and do some research on his own in the meantime - verrrry helpful guy!), it hit me like a TON OF BRICKS..... Both my bf and friend are not only iPhone users, but are on the OLD operating system because they (like many iphone users) didn't want to update to the new iOS because it apparently changed the look of the phone/apps. They are both in the process of installing and converting their iphone 4(s) - not "S" as in model but plural to the new operating system. Unfortunately, I can't report on whether this was the issue but I imagine with a brand new phone (HTC One M8), new system update that I installed yesterday and their old phone/VERY old iOS (this is like 2 versions ago - for both of them), there is a MAJOR incompatibility.
I will report back once they have installed and tried sending a pic, but hopefully this will help someone else for now (or who has a friend or significant other that can act quicker than mine do
)