Thank's for your answer. I've tried:
import arcpy
from arcpy import env
env.workspace = r"C:\ArcGIS\Projects\MyProject6"
arcpy.MakeFeatureLayer_management("myFc", "myFc_lyr")
inputLayer = "myFc"
symbologyLayer = "myFc.lyrx"
symbologyFields = [["VALUE_FIELD", "StrKlasse", "StrKlasse"],["VALUE_FIELD", "Abschnitt_Ast", "Abschnitt_Ast"]]
arcpy.ApplySymbologyFromLayer_management (inputLayer, symbologyLayer, symbologyFields)
All sources are in the same Project folder. The script runs and completes with no error, but nothing is changed.