From the error you posted it seems that seahub is trying to use modules inside the thirdpart folder of the server.
I worked around it by just renaming the PIL folder so that python can’t find the “wrong” PIL.
jobenvil pointed out in the same thread that pip3 install -U Pillow -t /media/sda/seafile/seafile-server-latest/seahub/thirdpart would be a better approach, installing Pillow to the thirdpart folder of seahub.