It appears that the problem is to do with an RSO function which calls event.on_tick, so it conflicts and breaks my code.
If anyone knows how to get around it, please do tell.

I'm modifying an Oarc scenario to add PvP and diplomacy between teams, however the on_tick event isn't being processed.
This is the code in question:
script.on_event(defines.events.on_tick, function(event)

-- Runs every 5 seconds
if (game.tick % 300 == 0) then
CheckDiplomacy ...