Why is camera still not working?

What the heck?! I have an htc One M8. The camera will not launch-still. I've called into tech support with htc 5 times; rebooted in safe mode, cleared cash, all that. I sent the phone in for repair, got it back, still experiencing the same exact problem. The camera will work if I restart the phone. However, if it has been on for a while and I try to start up the camera it will go black then go back to the home screen. I don't last pass on the phone - that I know of... Has anyone else had this problem? could it be another app that is interfering with the camera?PLEASE HELP!

Thank you :)