I’ve turned on debug output in logging and refreshed the calendar page a few times, here is what I see:
I looked at the calendar page in the nextcloud docs and it doesn’t mention separate logs, so it looks like there just aren’t any errors reported?
If I import the ical link to my phone’s ICSx5, the events do show up.
I also tested the ical file against https://icalendar.org/validator.html?url=https://www.federalpay.org/holidays/ical. The site finds an issue with line endings, but nothing else.