You weren't getting that attachment download failure message before?
If it's working normally in Safe Mode, that suggests that something you installed is causing the issue. The next thing I'd try is an app cache wipe, in case some temporary app data is causing a glitch. Go to
Settings>Storage, tap Cached Data, and then agree to clear it. If the problem still persists, then try a system cache partition wipe, which is different from the app cache wipe.
https://support.t-mobile.com/docs/DOC-21095
If the problem still persists, then you might have to uninstall apps one by one until the problem goes away. I'd start with any 3rd party apps that tinker with the system, like Clean Master (a very common culprit for various problems).