Hi @Wapata ,
did you try as OSC address just:
/CUIA/SCREEN_MAIN
@ToFF has made with success some research in OSC with Open Stage Control wich is a tool for designing remote control interfaces.
In his json configuration file, OSC addresses are like this and there is nowhere any network address like Zynthian.local:1370.
Also, did you try to monitor in/out coming messages ?
Maybe this helps.