iCal feed doesn't work: lots of public events, but "None found"
First I'd confirm that the functionality works on another site. You can use a demo site for this, or you can check Google for inurl:civicrm/event/ical



OK, so it works for some people! Next check might be to find out if the WordPress path works (inurl:civicrm/?page=CiviCRM&amp;q=civicrm%2Fevent%2Fical perhaps? IDK if Google / other search engines support checking for pages with specific query params though, as CiviCRM uses here.)

The next place I'd look would be what generates the iCal feed. Is it coming from an extension or core CiviCRM? Your question didn't say this for sure, so a search for "civicrm/event/ical" on the Git repo will reveal a menu XML entry if it's a core thing. (If not we'd go looking for the relevant extension.)

By searching the civicrm/civicrm-core repo I found that this is core functionality, because the path you're trying to understand has a menu.xml entry which shows up in the results.


https://github.com/civicrm/civicrm-core/blob/f6a3ccdf71f0127ca41b55c124b29915a0827e59/CRM/Event/xml/Menu/Event.xml


Looking at that we can see:


the permission checked for will be "view event info"
the page callback class is CRM_Event_Page_ICalendar


So you could perhaps add some debug code to that class (start at the run() method), and see if this method gets called when a visitor hits your iCal feed. 


https://github.com/civicrm/civicrm-core/blob/8b9a8f4abb25cfa7a498eaaa3e459e4d313e350a/CRM/Event/Page/ICalendar.php


There's also an open issue on CiviCRM's Gitlab, dev/wordpress#25. That's worth subscribing to, even though the description is different to yours ... the fact that both affected sites are WordPress may be significant.

Where you'd go from there depends on what you find so far, but your question was "where do I look next", so here you go! Good luck :)