I have a similar Problem, tried to curl it from the server and got the file, no issue there but from inside the plugin i hear:
SSL operation failed with code 1. OpenSSL Error messages:
error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed in /.../wp-content/plugins/ics-calendar/functions.php on line 390
then it sais failed to enable crypto and failed to open stream
not sure if this is related but i get this from google calendar as well as several nextcloud calendars
