I think I have been able to figure out why this was occurring:
Somehow I had a mod folder copied to another project folder within the Arma Reforger Tools/Addons folder.
(It can be triggered in any project (mod) folder and noot only the one you are attempting to open)
Because Arma Reforger Tools detected this out-of-place duplicate it causes this issue.
This was difficult to find. I only noticed it while I was trying to open any / all of my projects. Thanks