Mac mini, El Capitan, Plex stuck on 'please wait while media server starts for the first time'

Hi There
I have tried a number of things suggested in the forums (eg removing system and framework plugins) result is always the same. Plugin log directory is empty. Media server log is attached. This is from a run I did after removing and reinstalling Plex. An earlier install and attempt to start lasted several hours.
The system is recently upgraded from Yosemite to El Capitan.
Python seems to be installed ok but my knowledge of python is limited to running 'python' on the command line and reading the version.
I hope you can help me.
Jim