One of our servers had lost it's activation status and dropped it's product code. We tried the usual slmgr -ipk to reinstall the product code but that didn't work. Also tried the slmgr -upk, slmgr -cpk, slmgr -ato to no success. There was a slmgr switch that looked very promising, slmgr -rilc which should reinstall all the last known good keys. rebooting the server and trying to activate afterwards still didn't work...
Out of some obscure corner in Google I managed to find something that actually worked.
DISM /Online /Cleanup-Image /RestoreHealth
This apparently fixed a component store corruption (which probably was something to do with the Software Protection service). I then ran a sfc /scannow for good measure (probably not needed). Bounced the server and all was good.
The product code was showing up and we could then use slmgr /ipk to reinstall the KMS client and get our server activated. Remember to keep your servers happy, when they aren't all sorts of weird stuff goes down!
TLDR; Use 'DISM /Online /Cleanup-Image /RestoreHealth' to fix the component store corruption which broke the activation process.