Cannot update to 19.04 -the essential package 'ubuntu-minimal' could not be located.

Hi there; I’m having a problem updating 2 droplets, one from 18.04, the other from 18.10. Note that in the error below, it complains about not being able to find ubuntu-minimal, however, if I try and install it, it says:

“ubuntu-minimal is already the newest version (1.425.1)”

Here’s a summary of the output - most of the stuff about “No valid mirror found” seems fairly standard on each update and hasn’t been an issue before.

do-release-upgrade

Updating repository information

No valid mirror found

While scanning your repository information no mirror entry for the
upgrade was found. This can happen if you run an internal mirror or
if the mirror information is out of date.

Do you want to rewrite your 'sources.list' file anyway? If you choose
'Yes' here it will update all 'cosmic' to 'disco' entries.
If you select 'No' the upgrade will cancel.


Get:1 http://security.ubuntu.com/ubuntu disco-security InRelease [79.7 kB]
Hit https://repos.sonar.digitalocean.com/apt main InRelease
Get:2 https://deb.nodesource.com/node_8.x disco InRelease [4594 B]
Get:3 http://security.ubuntu.com/ubuntu disco-security/main Sources [712 B]
Get:4 http://security.ubuntu.com/ubuntu disco-security/main amd64 Packages [664 B]
Get:5 http://security.ubuntu.com/ubuntu disco-security/main Translation-en [380 B]
Get:6 http://security.ubuntu.com/ubuntu disco-security/main amd64 c-n-f Metadata [108 B]
Get:7 http://security.ubuntu.com/ubuntu disco-security/restricted amd64 c-n-f Metadata [116 B]
Get:8 http://security.ubuntu.com/ubuntu disco-security/universe amd64 c-n-f Metadata [112 B]
Get:9 http://security.ubuntu.com/ubuntu disco-security/multiverse amd64 c-n-f Metadata [116 B]
Get:10 https://deb.nodesource.com/node_8.x disco/main amd64 Packages [765 B]
Fetched 87.3 kB in 0s (0 B/s)

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done

Invalid package information

After updating your package information, the essential package
'ubuntu-minimal' could not be located. This may be because you have
no official mirrors listed in your software sources, or because of
excessive load on the mirror you are using. See /etc/apt/sources.list
for the current list of configured software sources.
In the case of an overloaded mirror, you may want to try the upgrade
again later.

Restoring original system state