When running a pipeline, the label expects a string with the option of an operator. Apparently the && operator with 2 groovy variables will evaluate correctly but not a || operator.

So this pipeline runs correctly:

def groovyLabel1 = 'master'
def groovyLabel2 = 'TestAgent'
node(groovyLabel1 && groovyLabel2){
echo "hello"
}

But this code does not run correctly:

def groovyLabel1 = 'master'
def groovyLabel2 = 'TestAgent'
node(groovyLabel1 || groovyLabel2){
echo "hello"
}

because it fails with this error:

java.lang.ClassCastException: org.jenkinsci.plugins.workflow.support.steps.ExecutorStep.label expects class java.lang.String but received class java.lang.Boolean

I think that groovy is doing some funky parsing here and causing the error because what should be the only thing allowed would be something like this:

node('master || TestAgent'){
echo "hello"
}