If the remote Ubuntu machine does not boot showing the following error

error: unknown filesystem
 Entering rescue mode...
 grub rescue>

the GRUB bootloader is broken and can be fixed in one of the following ways.

Using the GRUB rescue concole

Follow the steps described in this blog post.

Using a …

Assuming you are running an Apache server with a virtual host for abs.mydomain.com, put this into the virtual host's .conf file to redirect all traffic to a Jetty web server running locally behind your Apache:

<Proxy *>
    Order deny,allow
    Allow from all
</Proxy>

ProxyPass / http://localhost:1234/ retry …

Note to self: If the Let's Encrypt extension for Plesk fails to renew a certificate (when triggered manually), disable the automatic forwarding to an SSL connection in the Apache settings. This forced secure connection seems to disturb the renewal script.

The error I got was something like this:

Let's …