If you try to import an external calendar with the following URL...

webcal://www.mozilla.org/projects/calendar/caldata/AustraliaHolidays.ics

The form validation regards this as an invalid URL and prompts the user for another.

Replication steps:

1. Log in to Moodle as a student
2. Go to Navigation > Site pages > Calendar.
3. Click the button 'Manage subscriptions'.
4. Try importing a calendar with the URL webcal://www.mozilla.org/projects/calendar/caldata/AustraliaHolidays.ics

Expected result: The file should be used as the basis of a calendar

Actual result: Validation fails and user is asked to enter another URL

Note that the calendar import does accept the same URL with the http protocol.

http://www.mozilla.org/projects/calendar/caldata/AustraliaHolidays.ics

The problem is that the field for the calendar URL has the type PARAM_URL, which accepts only http, https, mailto and ftp.

The best solution would be to create some custom validation for this field that checks, if the text does not qualify using PARAM_URL, that it begins with webcal:// and the rest is a valid URL (perhaps substitute webcal for http and test).