I've found the issue. The problem was coming from the varnish's configuration, which was playing the set-cookie value.
NodeBB Upgrade caused issues
I have ran the code ./nodebb upgrade to upgrade to the latest edition, so that I can update my plugins which aren't compatible.
The upgrade doesn't seem to have worked (upgraded via Putty) and it keeps on re-circling the install.
my forum is now 503 Temp unavailable.
When i stop and start the service I get this error:
-bash-4.1# (node:12900) DeprecationWarning: Calling an asynchronous function without callback is deprecated.
When the Start command has been executed, my forum comes online before showing an error establishing a connection
I've found the cause, the Wordpress SSO plugin was causing issues.