SUMMARY
Adding an ICal calendar via URL or via a local file only creates empty calendars. 
Downloading the ICal file and double-clicking it in the file browser opens a dialog for importing it into KOrganizer which yields properly working calendar entries. 


STEPS TO REPRODUCE
1. Right click and add a new calendar 
2. Select "ICal Calendar File" 
3. Paste the URL to an ICal file in the file name field
4. Add a display name
5. Click okay

OBSERVED RESULT
Two empty calendars are created named "akonadi_ical_resource_file_#"  where # increases by 1 each time and "calendar". 

EXPECTED RESULT
One new calendar should be added with the specified display name. That new calendar should have all the entries that are in the specified file, be it local or by URL, and not be empty. 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.17.1-arch1-1
(available in About System)
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.3

ADDITIONAL INFORMATION
Downloading the ICal file and double-clicking it in the file browser opens a dialog for importing it into KOrganizer and either adding it to another calendar or making a new entry. Making a new entry creates a calendar with the name of the file as display name and with all entries properly working inside.