When I use protocolib's build #665, the vane-core plugin fails to load without any error, the server side is purpur1.20.1-2011, and the plugin version is 1.12.3
I verified that it loads fine for with newest ProtocolLib-665 and Paper-108, so either this is an upstream issue with Purpur or something else is wrong on your setup.
When I use protocolib's build #665, the vane-core plugin fails to load without any error, the server side is purpur1.20.1-2011, and the plugin version is 1.12.3