Well I tried the following:
$ date
Thu Feb 14 05:32:16 EST 2019
$ ntpdate ntp.ubuntu.com
14 Feb 05:26:58 ntpdate[12447]: step time server 91.189.91.157 offset -408.660323 sec
$ date
Thu Feb 14 05:27:02 EST 2019
So that was promising… but backup view still dead with the same error. So then I restarted the app:
$ ./launcher restart app
+ /usr/bin/docker stop -t 10 app
app
starting up existing container
+ /usr/bin/docker start app
app
Still no dice 
$ reboot
The system is going down for reboot NOW!
It’s a Digital Ocean droplet running vanilla Ubuntu. Comes back up… still not working and the time offset is gone:
$ ntpdate ntp.ubuntu.com
14 Feb 05:36:42 ntpdate[2375]: adjust time server 91.189.91.157 offset -0.001713 sec
I was convinced the 5 minute time correction would be key. Is it possible it’s just stuck now?
Any other ideas?
No. Separate bucket on S3 for backups; set-up unchanged for as long as anyone can remember.
So I couldn’t find anything interesting in logs on the host but i did look at the Discourse logs and noticed that it reports a slightly different error to the /admin/bakups.json
Failed to list backups from S3: The difference between the request time and the current time is too large.
Notice the extra prefix: Failed to list backups from S3