Seafevents fails to start when I try to switch to the professional server

The service(seafile & seahub) has been running successfully, and the most of function is normal except online file preview. When I check /logs/controller.log, I find seafevents service fail to start.

/logs/controller.log looks like this:

[08/24/18 15:29:35] seafile-controller.c(590): pid file /root/seafile/pids/ does not exist
[08/24/18 15:29:35] seafile-controller.c(616): seafevents need restart…
[08/24/18 15:29:35] seafile-controller.c(87): spawn_process: /usr/bin/python2.7 -m seafevents.main --config-file /root/seafile/conf/seafevents.conf --logfile /root/seafile/logs/seafevents.log -P /root/seafile/pids/
[08/24/18 15:29:35] seafile-controller.c(102): spawned /usr/bin/python2.7, pid 2343

When I try to run this in shell

/usr/bin/python2.7 -m seafevents.main --config-file /root/seafile/conf/seafevents.conf --logfile /root/seafile/logs/seafevents.log -P /root/seafile/pids/

It will show…

/usr/bin/python2.7: No module named seafevents

System: Debian 9
Seafile service version: seafile-pro-server_6.3.4_x86-64_Ubuntu

Switching sqlite to mysql will fix this problem.