Just to put a cap on this, for anyone who encounters this error, it looks like we had a couple things go wrong.
As far as the upgrade itself, the duplicate row in the “data_explorer” plugin was causing an issue and needed to be removed.
However as part of this we also ran an OS upgrade from Ubuntu 16 to 20, and this caused a networking error, rendering the Digital Ocean droplet unreachable on reboot. Specifically, it looks like as the OS moved to the new “Netplan” setup, something in the config was preventing the network interface boot script from completing at the point when it ran. So the Droplet came up, just not the network. Going in afterward, via the in-browser recovery console, and running the network interface script once more brought up the interface and allowed us to reach it from the outside again. So long as we don’t have to reboot the droplet, we’ll be good until we can find some time to power down and test a fix for the netplan configuration.
I know this is probably an edge case, but I remember reading somewhere that OS upgrades generally don’t cause issues. And while that is probably true most of the time, that wasn’t the case here, and kept us offline for a good half a day until the time zones aligned to where our technical hired muscle could get us back up again.
So, all resolved for now. Thanks for the speedy input everyone.