I export my NextCloud calendars to .ICS format to use with MagicMirror on a raspberrypi.

I see in some of the .ICS entries items like the following:


My NextCloud calendar entries probably originated from my old Google calendar a long time back. They may have been in my Google calendar, with an invite sent to the NextCloud calendar. From there they may then have been edited; I since recycle my recurrent appointments in my NextCloud calendar for a school timetable (so edit and shift them forward every few months, for the forthcoming term). I no longer use Google calendar.

How do I find where such google references are stored in the NextCloud calendar, and how do I remove them?

You can export the whole Nextcloud calendar to an ics file, edit that file and re-import it into Nextcloud.