Can't seem to update Ubuntu 14.04.1 (Trusty) to 14.10 (Utopic) - source network errors.

Just trying to upgrade my Ubuntu as I’ve done many times before, but I’m coming up with a network error here at DigitalOcean. Here’s what’s happening just after the normal warning about the newly created port…

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Err http://mirrors.digitalocean.com trusty/main Translation-en_US
Err http://mirrors.digitalocean.com trusty/multiverse Translation-en_US
Err http://mirrors.digitalocean.com trusty/restricted Translation-en_US
Err http://mirrors.digitalocean.com trusty/universe Translation-en_US

Ign http://mirrors.digitalocean.com trusty/main Translation-en_US
Ign http://mirrors.digitalocean.com trusty/multiverse Translation-en_US
Ign http://mirrors.digitalocean.com trusty/restricted Translation-en_US
Ign http://mirrors.digitalocean.com trusty/universe Translation-en_US
Fetched 0 B in 6s (4,153 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done

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if the mirror information is out of date.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 'trusty' to 'utopic' entries.
If you select 'No' the upgrade will cancel.

I enter “Yes” and it does pretty much the same thing again, and then…

Error during update

A problem occurred during the update. This is usually some sort of
network problem, please check your network connection and retry.

W:Failed to fetch
http://ppa.launchpad.net/jon-severinsson/ffmpeg/ubuntu/dists/utopic/main/binary-i386/Packages
404 Not Found
, W:Failed to fetch
http://linux.dropbox.com/ubuntu/dists/utopic/main/binary-i386/Packages
404 Not Found
, E:Some index files failed to download. They have been ignored, or
old ones used instead.

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done
=== Command detached from window (Fri Oct 31 06:14:37 2014) ===
Press x to destroy or r to resurrect window  i Oct 31 06:14:47 2014) ===

Those last couple of packages shouldn’t matter; missing third party stuff has never blocked and install before. I’m more interested in the missing DigitalOcean mirror stuff.

Yes, I’ve rebooted, and yes I’ve checked that the droplet has network connectivity (obviously, as I wouldn’t be connected to it!).

Any ideas?