This post has been updated to summarize the issue. The original reply to Simon is included at the bottom of this reply for posterity.
We have added a warning to the authorization flow that will show up if the Hubitat OAuth servers are slow to respond. That warning links to this thread.
As alluded to in that warning, the Hubitat OAuth servers have been experiencing significant performance issues. We created a post in the Hubitat community and have been tracking the issue with the Hubitat team:
Unfortunately, this issue impacts any service which similarly authenticates with Hubitat including Google Home, Alexa, IFTTT, and more.
We significantly increased the request timeout on our side to try to give the OAuth servers more time to respond, but if they don’t respond within 60 seconds (which is an exceptionally long time to wait in the modern internet), the process will timeout.
If you already have your Hubitat location authorized, you can update your device selections from the SharpTools ‘app’ settings on your Hubitat hub. This bypasses the Hubitat OAuth servers.
Some people have found that if they wait 5 minutes and then try again, sometimes they can get things to go through. The performance of the Hubitat OAuth servers seems to degrade more and more over time, so sometimes this requires retrying several times over a long period of time before it works.
Background device sync improvements (tap to expand)
We’ve also update our background device sync process to workaround this issue for users with existing Hubitat connections. This will not fix things for the primary authorization for as that fundamentally relies on the Hubitat OAuth servers, but will workaround things for background device syncs that may occur as you edit your dashboards or rules.
Previously, the background device sync process needed to ask the Hubitat OAuth servers for a list of locations – unfortunately, because the OAuth servers were experiencing significant issues, this could cause the locations to become deauthorized from SharpTools. We’ve added logic that will fallback to querying our internal location snapshot if the Hubitat OAuth servers don’t respond in time.
Original reply to simon (tap to expand)
I’m seeing the same thing when I try to reauthorize as well… it’s interesting as that message does in fact come from Hubitat’s servers directly. So it’s weird that they thing the hub is offline when I’m able to use Cloud URLs from other ‘apps’ like the Hubitat Maker API. 
I get the same error message when trying to authorize other Hubitat OAuth apps like IFTTT, Alexa, and Google Home, so it seems like there’s something funky going on at the HE side of things…
Edit: Here’s a link to the post I created in the HE Community:
[RECURRING] OAuth Connections Down / Performance Issues - 🚧 Developers - Hubitat