I’ve also tried this with my Nextcloud calendar and it doesn’t work in Obsidian, although the link works fine in Firefox and Edge.
It seems that Nexcloud blocks the use of iframes. I found this thread on what to change in Nextcloud to allow iframes: Freigegebenen Kalender in iframe anzeigen - #3 by KlausK - 🇩🇪 Deutsch (german) - Nextcloud community