Thanks @Justin_B!
Your suggestion put me on the right track and now it seems to be working OK!
Solution:
There wasn’t a system environment variable pointing to the Plugins folder at all.
This might have been the problem in the first place, so I looked on the Autodesk documentation and after coming across a post from Changsoo explaining how to use the new variables available I’ve added a new environment variable set up like this:
Variable Name: ADSK_3DSMAX_PLUGINS_ADDON_DIR
Variable Value: C:\Program Files\Autodesk\3ds Max %ADSK_3DSMAX_MAJOR_VERSION%\Plugins
%ADSK_3DSMAX_MAJOR_VERSION% should allow the plugins to be loaded correctly as its value should change according to which version of max is launched by Deadline.
I can keep max 2023 with Corona 9 and max 2025 with Corona 12.
It seems Vray has its own environment variable already set-up.
Thank you so much for your help!