Sorry it took so long to look back into this.
Your tip worked perfectly, thank you so much! I even got the login background image to work by chmod-ing the whole seahub-data folder.
I also double-checked the owner and groups and made sure they were all well and set to root:root, as a few sections were set to root:nogroup, due to my NFS server settings.
Here are the commands I ran:
# /SEAHUB-DATA : recursively for avatars and login background images
chmod -R 755 seahub-data
chown -R root:root seahub-data
# /SEAFILE
chmod 755 seafile
chown root:root seafile
# /SHARED : bind mounted "data" folder
chmod 755 data
chown root:root data