Agent upgrade failed: Unknown error

In particular, where the last lines of the upgrade log read:

Arch detected: armhf
Sending zipped agent
Agent upgrade failed: Unknown error

This probably means that there is a certificate error on a older Raspberry Pi (armhf). To confirm this is the problem, head to REMOTE SHELL and run

curl https://app.ardexa.com/version

If you get a "SSL certificate problem", then please run the following commands to update the trust store.

cp /usr/share/ca-certificates/mozilla/* /usr/local/share/ca-certificates/;
update-ca-certificates

This will update the certificates with the more recent mozilla certificates. To confirm the fix has worked, simply run the curl command again.