I’m not sure it’s the correct way, but my workaround worked for me…
I renamed the folder (in your case)
/home/seafile/installed/seahub/thirdpart/PIL/
to something like
/home/seafile/installed/seahub/thirdpart/PIL.old (in order to be able to restore everything).
This way, python isn’t finding PIL inside the seafile server installation files and looks for the user/system-installed version.
You can give it a try, for me it worked. And if it doesn’t work, just re-rename your folder and nothing changed…