You may be able to clear these warnings by clearing the cache so your calendars rebuild. Go to ICS Calendar > Utilities > Clear Cached Calendar Data.
You probably also want to turn off WP_DEBUG, in general, if this is a live site. It requires directly accessing your WordPress files by FTP or SFTP. The wp-config.php file should have this line:
If it says “true”, change it to false.
I haven’t seen these warnings come up in testing, but I’ll investigate further.