Maybe start here, the Wiki suggests a few ways to access the system without the webconfig
Command Line User Guide - ZynthianWiki
Also this topic:
Log file and level - Development - Zynthian Discourse
@3tree : This seems to be the one you are looking for journalctl -u zynthian-webconf
You can tail it if you want to see the last few lines output instead of scrolling through the whole log as well