How can appointments from OptiOffice be imported into another calendar?

Applications tested:

Lightning Add On for Thunderbird:
Create a new calendar -> in the network -> iCalender (ICS) -> enter the desired url under address -> select name and color, empty email, done. The calendar is now displayed and updates according to the normal synchronization settings.

Google Calendar:
In the web interface of the Google calendar in the menu next to -more calendars- select the item -add via url-. Finished. The synchronization of the calendar is in the hands of Google, but apparently happens every 6-24 hours.

Outlook:
Calendar view -> Start menu -> Open dropdown under Calendar -> From the Internet -> Enter URL -> Confirm -> Done. Synchronization according to Outlook settings.

Android Apps:
The calendar apps use the Google account. A calendar set up there also appears here.

iOS Mobile:
If the calendar is connected to a provider account (e.g. Google) it can easily be synchronized via this account. But you can also do without these accounts: Settings -> Mail, Contacts, Calendar -> Other -> Add calendar subscription -> Enter url -> done. The synchronization can be set under data synchronization.

App Kalender iOS:
If the calendar is connected to a provider account (e.g. Google) it can easily be synchronized via this account. But you can also do without these accounts: Open calendar -> File -> New calendar subscription -> Enter url -> Subscribe. The location can then be specified in the next window (local for only the computer, account to save it there and, if necessary, to integrate it more easily on other devices), as well as the frequency of the synchronization. OK Finishes the setup.