Network oddities - what's going on?!

So I have the following setup:

FTTC Internet connection comes into VDSL modem, which then connects to my Airport Extreme Base station. To that I have connected:

x1 wired airport express ("Kitchen")

x1 wireless airport express ("Bedroom")

x1 wireless time capsule ("Ed's AirPort Time Capsule")

Recently, the time machine has stopped backing up, saying disk cannot be found. Airport Utility is showing the following:

However, I can browse to the time capsule no problem from my Mac, and data saved on it (video files) can be played from my MPB, iPhone, iPad...any other device! Apart from the time machine, everything jsut seems fine. I'm really confused why the "Base" device is showing the exclamation mark, when it's working fine...hovering over it gives this:

But like I say, it seems to be fine!

I'm really reluctant to "Forget" it as there are all sorts of IP reservations, port forwards etc set up. Will forgetting it lose those?

I've tried powering everything down, disconnecting everything and connected one by one...but I just end up with the same problem.

Any advice as to what's going on? I've now stupidly removed the backup disk from the Time Machine, and it can't find it to re-add. But like I say, I can see it in finder under "Shared" fine and read / write files to it.

All devices up to date (Sierra, iOS 10.2)

Thanks all.

Ed