Problems upgrading from 2021.7.4 to 2021.10 - HA not restarting

I run HA Core in a Docker container on a Lubuntu VM. I’ve been running v2021.7.4 for the last few months, and decided to try upgrading to the latest stable release, so I followed the instructions here to upgrade:
https://jackstromberg.com/2020/03/how-to-update-home-assistant-docker-container/

Now HA does not seem to be starting up correctly - I’m unable to connect to the Lovelace interface. I attach what I believe to be most important parts of the HA log from the upgrade.
There seems to be a problem with the SSL certificate which has prevented the http element from being loaded (and which in turn has caused countless other components to fail to load). My v2021.7.4 installation was working perfectly, so I don’t know what the problem is with the SSL cert. This is my config for it:

http:
  ssl_certificate: ssl/fullchain.pem
  ssl_key: ssl/privkey.pem

I can confirm that both the fullchain.pem and privkey.pem files are in the ssl subfolder of my HA config folder.
Any idea why the error might be arising?

2021-10-11 17:46:27 ERROR (MainThread) [homeassistant.config] Invalid config for [http]: not a file for dictionary value @ data['http']['ssl_certificate']. Got 'ssl/fullchain.pem'
not a file for dictionary value @ data['http']['ssl_key']. Got 'ssl/privkey.pem'. (See /config/configuration.yaml, line 7). Please check the docs at https://www.home-assistant.io/integrations/http
2021-10-11 17:46:27 ERROR (MainThread) [homeassistant.setup] Setup failed for http: Invalid config.
2021-10-11 17:46:27 ERROR (MainThread) [homeassistant.setup] Unable to set up dependencies of system_log. Setup failed for dependencies: http
2021-10-11 17:46:27 ERROR (MainThread) [homeassistant.setup] Setup failed for system_log: Could not set up all dependencies.
2021-10-11 17:46:27 WARNING (Recorder) [homeassistant.components.recorder.util] The system could not validate that the sqlite3 database at //config/home-assistant_v2.db was shutdown cleanly
2021-10-11 17:46:27 INFO (MainThread) [homeassistant.setup] Setup of domain recorder took 0.2 seconds
2021-10-11 17:46:27 INFO (MainThread) [homeassistant.bootstrap] Setting up stage 1: {'api', 'onboarding', 'auth', 'person', 'system_log', 'image', 'webhook', 'lovelace', 'frontend', 'cloud', 'config', 'search', 'device_automation', 'analytics', 'http', 'websocket_api'}
2021-10-11 17:46:28 INFO (MainThread) [homeassistant.setup] Setting up lovelace
2021-10-11 17:46:28 INFO (MainThread) [homeassistant.setup] Setting up device_automation
2021-10-11 17:46:28 INFO (MainThread) [homeassistant.setup] Setup of domain device_automation took 0.0 seconds
2021-10-11 17:46:28 ERROR (MainThread) [homeassistant.setup] Unable to set up dependencies of api. Setup failed for dependencies: http
2

I worked it out - my SSL certs were actually being used from this folder:

/etc/letsencrypt:/etc/letsencrypt

…so I ran

sudo docker rm home-assistant
sudo docker run -d --name="home-assistant" --restart unless-stopped \
-v ~/homeassistant:/config -v /etc/letsencrypt:/etc/letsencrypt \
-v /etc/localtime:/etc/localtime:ro --net=host homeassistant/home-assistant

BUT…now I notice that the version number is still showing as 2021.7.4 in Lovelace.

???