A user managed to create an event with an invalid RDATE using roundcube and libcalendaring in Kolab:

BEGIN:VEVENT
UID:BDF720338B3EC8D2C09CE95CFF85C0B1-A26036CE0B219773
DTSTAMP;VALUE=DATE-TIME:20151211T121402Z
CREATED;VALUE=DATE-TIME:20151013T061627Z
LAST-MODIFIED;VALUE=DATE-TIME:20151014T093742Z
DTSTART;VALUE=DATE-TIME;TZID=Europe/Berlin:20151028T124500
DTEND;VALUE=DATE-TIME;TZID=Europe/Berlin:20151028T141500
SUMMARY:test summary
DESCRIPTION:test desc
SEQUENCE:1
RRULE:FREQ=WEEKLY;INTERVAL=1;UNTIL=20160127T114500Z
EXDATE:20151104T114500Z
EXDATE:20151118T114500Z
EXDATE:20151202T114500Z
EXDATE:20151216T114500Z
EXDATE:20151223T114500Z
EXDATE:20151230T114500Z
EXDATE:20160106T114500Z
RDATE;VALUE=DATE:20151028,20151111,20151125,20151209,20160113,20160120,2016
0127
TRANSP:OPAQUE
CLASS:PUBLIC
ORGANIZER;CN=:mailto:sb@somewhere.net
END:VEVENT
END:VCALENDAR

According to RFC5545:
The "DTSTART" property
value SHOULD match the pattern of the recurrence rule, if
specified. The recurrence set generated with a "DTSTART" property
value that doesn't match the pattern of the rule is undefined.

Obviously that is not the case. Its missing the time. I first tought this is a SabreDav problem but it looks like its not: https://github.com/fruux/sabre-vobject/pull/281