Error while resizing image: "error: (-215:Assertion failed) func != 0 in function 'resize'"

Oh, I actually figured it out. Images in the dataset were of type numpy.int64. I just had to convert images to float32, like this:

def resize_dataset(images):
    resized_images = []
    for img in images:
            img = img.reshape((28,28)).astype('float32')  # <-- convert image to float32
            resized_img = cv2.resize(img, dsize=(10, 10))
            resized_images.append(resized_img)
    return numpy.array(resized_images)

And now it works nicely. It looks like cv2.resize can't work with images represented in int. Hope this will help anyone