Thanks for the URL. The problem are these lines:
ACKNOWLEDGED seems to be defined by this draft: https://tools.ietf.org/id/draft-daboo-valarm-extensions-04.html#rfc.section.8.1 and is understood and parsed by ical4j.
ACKNOWLEDGED is defined like this:
acknowledged = "ACKNOWLEDGED" acknowledgedparam ":" datetime CRLF
acknowledgedparam = *(
; the following is OPTIONAL,
; and MAY occur more than once
In your iCalendar resource, there are two colons (:), which is not parseable.
So, this is an invalid iCalendar file. Please fix the your iCalendar or report this problem (including this analysis) to someone who can fix it.