Looks like you've downloaded the src files instead of just the dll. The src zip I attached is just for the developer and/or people who want to compile the plugin themselves, and includes several files that Launchbox will recognize as plugins and try to load them, causing the above error, to fix, remove everything from the plugin folder except for the 'assets' folder and settings.ini, then download the single dll. your PCSX2 Configurator Next folder should look like this:
Looks like that was the issue, only needed to change one line of code to get it working again with the new paths. I've fixed the issue using the dev's github source code. compiled plugin and src attached
PCSX2 Configurator Next-fixed.zip
PCSX2 Configurator Next.dll