For the last month or two I've been running into an issue where my AirPort Time Capsule (latest model) keeps requesting to create a new backup after a week or two. To give some info about the device, it's essentially a brand-new device as I just got it back around September. It's set in wired mode, as I have wireless outputted via an existing Airport Extreme, so it's just being used as a ethernet switch hub essentially right now (and for backups of course). Since my iMac and MacBook Pro are encrypted, I have backups set to be encrypted as well if that changes things at all. I do notice if I check AirPort Utility that after a day or two it will not show up in there (i.e. it does the "can't find it in your network, do you want to remove it"), but I can still backup to it until this issue occurs again. I can't recall if this happened before or after the Sierra update (on the latest version, 10.12.1), though. So it may have possibly been due to that but I'm not 100%.