Malfomed Graphdef error.

I create a MLP model as follows but get error.

import tensorflow as tf
from tensorflow.keras.datasets import mnist
from tensorflow.keras.models import Sequential
from tensorflow.keras.layers import Dense, Flatten
from tensorflow.keras.callbacks import TensorBoard
import datetime

(x_train, y_train), (x_test, y_test) = mnist.load_data()

x_train, x_test = x_train / 255.0, x_test / 255.0

model = Sequential([
    Flatten(input_shape=(28, 28)),      
    Dense(128, activation='relu'),      
    Dense(64, activation='relu'),       
    Dense(10, activation='softmax')     
])

model.compile(
    optimizer='adam',
    loss='sparse_categorical_crossentropy',
    metrics=['accuracy']
)

log_dir = "logs/fit/" + datetime.datetime.now().strftime("%Y%m%d-%H%M%S")
tensorboard_callback = TensorBoard(
    log_dir=log_dir,
    histogram_freq=1,       
    write_graph=True,       
    write_images=True,      
    update_freq='epoch'     
)

try:
    model.fit(
        x_train, y_train,
        epochs=5,
        validation_data=(x_test, y_test),
        callbacks=[tensorboard_callback],
        verbose=1
    )
except Exception as e:
    print(f"error when train: {str(e)}")
    raise

try:
    model.save('mnist_mlp_model.keras')
except Exception as e:
    print(f"error when save: {str(e)}")
    raise

print(model.summary())

My environment info is as follows.
macos 15.5 (24F74)
python 3.9.6
tensorboard 2.19.0
tensorboard-data-server 0.7.2
tensorflow 2.19.0
tensorflow-io-gcs-filesystem 0.37.1

Start by running the supplied TensorFlow MNIST script with the listed macOS, Python, TensorBoard, and TensorFlow versions, then inspect the full traceback behind the “Malformed GraphDef” error rather than relying on the screenshot. Trace the failure from the TensorBoard callback used during model.fit; done means the cause and affected behavior are confirmed and the issue has a reproducible resolution.