Tracking units going to unknown state

I have 30 units on traccar. every week or so , about 5 units go to unknown state and I have to restart the server, then only the unknown unit pick up. I have checked the units settings are all seem to be correct. I'm using version 4.6. Any ideas. I am using mysql database on the same server.

I would recommend to start troubleshooting by checking logs.

I have the same problem in demo 1 server with all devices