Alexander Samoylov GroovyInstallation is Java code from a plugin. Compared to Groovy code in Pipeline, the most important differences are that it is not CPS-transformed, it is not sandbox-transformed, its class loader hierarchy is different, and there is a class file on disk for it somewhere instead of whatever happens for Groovy code run by the embedded Groovy interpreter. I suspect the latter is what is causing problems in your case. Both Pipeline Groovy code and remoting do some tricky low-level things, and so they don't play well together.
If you just want to run some Groovy code for debugging purposes, you could try using the script console, which will at least eliminate the Pipeline-specific complexities, to see if that makes a difference.