Elastic agent wont start fleet server after upgrade

I represent an Australian school that uses Elastic for logging and we really love the product.

We have a single ELK server with a single ELK agent and Fleet server all on the same host running 8.10.1

After a recent upgrade of the ELK stack and Fleet server, the fleet server never restarted, it now just hangs in (STARTING) mode forever with no logs giving any indication of why.

┌─ fleet
│ └─ status: (STARTING)
└─ elastic-agent
└─ status: (HEALTHY) Running

In the GUI it simply shows as offline.

I've tried updating the Agent to a fairly recent version (8.12.2) and this hasn't assisted. I'm relucent to un-install and re-install as I have a number of integrations that were challenging to setup.

Can any please give any advise on how I can troubleshoot this? I've spent two hours going through forum posts and haven't been able to find anything relevant on where to start.