Invalid session token - Stopping server

I followed the guide "Production Install on Ubuntu 14.04 LTS" and I have successfully installed it (3.4.0) in one machine configuration.
Everything seems to works, but after a while the server has started, the mattermost service always stops working. I have to restart the server to see mattermost working again.
Here is the log:
[2016/10/09 22:50:59 CEST] [INFO] Server is initializing...
[2016/10/09 22:50:59 CEST] [INFO] Pinging sql master database
[2016/10/09 22:50:59 CEST] [INFO] Pinging sql replica-0 database
[2016/10/09 22:51:01 CEST] [WARN] Unable to find active license
[2016/10/09 22:51:01 CEST] [INFO] Starting Server...
[2016/10/09 22:51:01 CEST] [INFO] Server is listening on :8065
[2016/10/09 22:51:01 CEST] [INFO] RateLimiter is enabled
[2016/10/09 22:55:32 CEST] [EROR] Invalid session token=475yuw79m7rwfy8qrkder8t1zh, err=sessionIdOrToken=475yuw79m7rwfy8qrkder8t1zh
[2016/10/10 07:44:51 CEST] [INFO] Stopping Server...