If you want to display additional custom log files in the WebGUI at 'Diagnostics | System Logs' then you have to do the following. Create a file called 99mylogspecs.inc in /usr/share/openmediavault/engined/inc. Here you can add your log file specifications.
The description of the structure can be found here:
-> monit restart omv-engined
During development you should manually start the engine to see if there are any errors. They will be displayed immediatelly.
-> omv-engined -d -f
THX to votdev at omv-forum