I think your best bet is to manually start Zynthian and see the logs in the terminal. Also, setting the log level to 10 may draw more information. I’m not near my unit, so I can’t tell you exactly how. Let me know if you didn’t find it. 
EDIT: Maybe something like this:
systemctl stop zynthian
cd /zynthian/zynthian-ui
export DISPLAY=:0
export ZYNTHIAN_LOG_LEVEL=10
./zynthian_main.py
Mmm, thinking of it, maybe something is missing, because the Zynthian service starts the X server, and so you won’t have one. In my case, I use Xnest on my host, and then connect to Zynthian using SSH with X forwarding, and that’s why it works for me…