I 'upgraded' to the V5 firmware on my 7616-M2 series NVR and ran straight into the failed camera stream syndrome where all the streamed cameras fail to be recognised. (I don't use the NVR POE ports at all as all my cameras are collected by a remote gigabit POE switch elsewhere). As someone else has mentioned, the good news is that you can revert back to the 'old' v4 firmware*, (personally I preferred the old v3 firmware, but that's progress..). The bad news is you'll find that you have to reactivate your NVR, (losing all your settings in the process), and re-initialise your hard drive(s) so you lose all your recordings and have to reinstall all your cameras too. What a dog's breakfast... I suspect that it is probably down to something simple like the NVR ending up on the 'wrong' network IP address after the v5 'upgrade'. When I've recovered my strength and my hair regrows I'll try again, but for anyone else - make sure your v5 upgraded NVR is on the same network as your cameras and does that sort the problem of the camera stream not being recognised?
Apologies if someone else has posted the above 'one simple trick' answer elsewhere, but I couldn't find it...
*v4 firmware - if you previously downloaded it you should find that the old v4 firmware is still in your downloads folder.