local file is not getting imported

I've used the local file URL: file:///tmp/test.ics and ensured that the file is located in /tmp/test.ics but still get the error message that the file could not be imported from the given URL "The URL is not valid, could not download data."
Unfortunately the official url from https://albdonaukreis.buergerportal.digital/calendar/ does not support the download for the locations.