This morning I added first a new task, then a new event via KOrganizer with basically the same content. After that I get calendar errors, and my main calendar simply wont load 
In the calendar UI the calendar entry just has a “spinner”, and the console gives: Error: Same occurrence found twice, protecting you from death by recursion vendor.js:41295:14.
I tried removing cache and cookies for my domain, but that didn’t help.
There is nothing related in the NC log.
I’m on NC 17.0.1using PHP 7.3.11, MySQL 10.2.29 and Apache, dunno which version, as I’m on shared hosting.
I can download the calendar from the web UI, so all is not lost, and I can sync from phone to PC via NC without problems. I can’t sync the other way, but that is most likely due to the crappy CalDAV support in SFOS. I was wrong, sync doesn’t work
Anybody experiencing the same? Found an issue reported November 2019 https://github.com/nextcloud/server/issues/17855