I've been having this problem for about a week with one of my four cameras and its been driving me nuts. I even created an RMA request for it yesterday.
I finally figured It out today by pure luck though. It turned out to be an ip address conflict. I have ip addresses reserved in my router for all my cameras and some other stuff. Today I happened to notice my NAS was using the ip address I have reserved for that camera.
I have no idea why my router gave it that IP address but I rebooted my NAS to make it request an address again and everything's fine now.