I have recreated what you want with the code you gave and got this to work
import sys
import os
from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *
class Wnd(QMainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.initUI()
        self.menuoffnen()
        pass
    def initUI(self):
        self.setGeometry(200, 200, 800, 600)
        self.layout = QVBoxLayout()
        self.layout.setContentsMargins(0, 0, 0, 0)
        self.setLayout(self.layout)
        self.texteditor = QPlainTextEdit('')
        font = QFont()
        font.setPointSize(12)
        self.texteditor.setFont(font)
    
        #self.layout.addWidget(editormenu) # Editormenü
    
        self.setCentralWidget(self.texteditor) # Eingabefeld für Texdateien
        self.setWindowTitle("NodeEditor")
        self.show()
    def menuoffnen(self):
        dateiname = QFileDialog.getOpenFileName(self, 'Datei öffnen','','Textdateien (*.txt)')
        dateiname = os.path.abspath(dateiname[0])
        dateininhalt = ''.join(open(dateiname, encoding="utf-8").readlines())
        self.texteditor.setPlainText(dateininhalt)
        self.setWindowTitle(os.path.basename(dateiname) + " - Marlems PQTTexteditor")
if __name__ == '__main__':
    app = QApplication(sys.argv)
    wnd = Wnd()
    sys.exit(app.exec_())
I have tried it with a simple txt file and it worked without errors. Could you test it with your file?
I also find that you forgot to pass self as a parameter into menuoffnen, or is it again a problem in the post?