Currently using:

• LOS 16 with Security Updates October 2019
• version 3.20 BETA 2(checked: same error with version 3.20)
• log service enabled
• no error if log service is disabled (checked)

If log serice is enabled log file is empty.

Solution: switch back to 3.10, log works as expected. No error on boot.

Log:
10-20 00:36:44.706 E/AndroidRuntime( 2301): FATAL EXCEPTION: main
10-20 00:36:44.706 E/AndroidRuntime( 2301): Process: dev.ukanth.ufirewall, PID: 2301
10-20 00:36:44.706 E/AndroidRuntime( 2301): android.app.RemoteServiceException: Context.startForegroundService() did not then call Service.startForeground(): ServiceRecord{30ad649 u0 dev.ukanth.ufirewall/.service.LogService}
10-20 00:36:44.706 E/AndroidRuntime( 2301): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1745)
10-20 00:36:44.706 E/AndroidRuntime( 2301): at android.os.Handler.dispatchMessage(Handler.java:106)
10-20 00:36:44.706 E/AndroidRuntime( 2301): at android.os.Looper.loop(Looper.java:193)
10-20 00:36:44.706 E/AndroidRuntime( 2301): at android.app.ActivityThread.main(ActivityThread.java:6718)
10-20 00:36:44.706 E/AndroidRuntime( 2301): at java.lang.reflect.Method.invoke(Native Method)
10-20 00:36:44.706 E/AndroidRuntime( 2301): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
10-20 00:36:44.706 E/AndroidRuntime( 2301): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)