For me the seafile client didn’t initially work on Ubuntu 18.04 and I spent 3 days now searching forums to find a solution which worked for me.
The problem was that when trying to start seafile-applet there came an error message:
This application failed to start because it could not find or load the Qt platform plugin “xcb”.
I tracked the problem down to be related to a missing dependency of the file libqxcb.so (ldd libqxcb.so) - the libxcb-xinerama.so.0 was not found.
What solved it for me was the command described in this forum: https://ubuntuforums.org/archive/index.php/t-2391244.html
sudo apt install --reinstall libxcb-xinerama*
Maybe this will help somebody…