New Calendars don't appear in clients


recently, I’ve imported a calender at Nextcloud server in the calendar app. That worked. But however, no matter at which client I update the list of available calendars, the new one doesn’t show up. Even though the validator says it has one error, it still appears in Nextcloud server.
I’ve tested it in Android using Davx 5 and in Thunderbird on Linux Desktop (directly, since Thuderbird offers this feature directly without the need of the Lightning extension)

To be concrete, it’s an Apple iCalendar subscription of a friend of mine, so I got read-only permissions on that calendar subscription.

May you tell me please, why the new calendar won’t show up on any client?

Some technical notes you might need to know:
Nextcloud Version: 23.0.5
calendar app version: 3.5.7
Server: bare metal instance, only one Nextcloud server instance, based on openSUSE Leap 15.3 with nginx php 7.4 and fpm
I’ve checked the Nextcloud logs, no log entry for calendar issues found.
I’ve waited one day and nothing has changed on my clients, still not listed in calendars (in order to exclude cronjob executions as a problem).

A side note: I’ve also got that problem a while ago when I switch my main mobile phone. There I got also troubles to receive all calendars (for whatever reason, it should simply work). Also, creating new calendars which are straight Nextcloud calendars also take a while to be visible on clients. So, if there’s a bug in the calendar app, it’s not only related to the latest version of it, it exists for a while now.

Thanks in Advice,