From https://issues.kolab.org/show_bug.cgi?id=3049:
If a VEVENT object has a full URI as its unique identifier, invalid CalDAV URLs for these objects are created and thus synchronization fails. For example, events from the f1calendar.com feed look like this: SUMMARY:Australian Grand Prix UID:http://2014.f1calendar.com#GP1_2014 In a multistatus request, iRony produces invalid URIs like this: <d:href>/calendars/john.die%40mykolab.com/ff228802b9-53dabd4f3356-f270fcdd4/http://2014.f1calendar.com#GP1_2014.ics</d:href> which is wrongly encoded and when a client wants to fetch such an event a "Object not found" exception is returned.
The original fixes are in rI4561e85 (then master, now 0.3) and rIe916334 (0.2), and should have been resolved as per version 0.2.9.
It cannot be reproduced on current master (0.4), but it seems to occur on 0.3.0 (caldav.kolabsys.com) but not on 0.3 (developer workstation).