I have tried to compare your config to mine. I don’t actually know what’s wrong here, but I had a similar problem that I finally fixed. Here are a couple of differences I noticed between your config and mine:
That is the wrong slash. Try:
FILE_SERVER_ROOT = 'https://file.example.com/seafhttp'
I also have these extra lines you don’t have:
ALLOWED_HOSTS = ['.example.com']
# Whether to use a secure cookie for the CSRF cookie
# https://docs.djangoproject.com/en/3.2/ref/settings/#csrf-cookie-secure
CSRF_COOKIE_SECURE = True
# The value of the SameSite flag on the CSRF cookie
# https://docs.djangoproject.com/en/3.2/ref/settings/#csrf-cookie-samesite
CSRF_COOKIE_SAMESITE = 'Strict'
# https://docs.djangoproject.com/en/3.2/ref/settings/#csrf-trusted-origins
CSRF_TRUSTED_ORIGINS = ['https://file.example.com']