Not sure if this will help anyone, but I started receiving this error a few days ago. The error would not allow me to add new events, invite attendees or allow other to accept invitations. I started console, and tried to add an event.... this is the report...
6/11/11 10:00:49 AM iCal[1039] CalDAV CalDAVWriteEntityQueueableOperation failed: status 'HTTP/1.1 400 Bad Request' request:
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Apple Inc.//iCal 4.0.4//EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
CREATED:20110611T140045Z
UID:**THIS UID WAS REMOVED**
DTEND;VALUE=DATE:20110621
TRANSP:TRANSPARENT
SUMMARY:test
DTSTART;VALUE=DATE:20110620
DTSTAMP:20110611T140047Z
SEQUENCE:2
END:VEVENT
END:VCALENDAR
... response:
HTTP/1.1 400 Bad Request
Date: Sat, 11 Jun 2011 14:00:49 GMT
Dav: 1, access-control, calendar-access, calendar-schedule, calendar-auto-schedule, calendar-availability, inbox-availability, calendar-proxy, calendarserver-private-events, calendarserver-private-comments, calendarserver-principal-property-search
Content-Type: text/html
Content-Length: 285
Server: Twisted/8.2.0 TwistedWeb/8.2.0 TwistedCalDAV/2.5 (iCal Server v12.58.7)
<html><head><title>Bad Request</title></head><body><h1>Bad Request</h1><p>URL may not contain a query or fragment: /calendars/__uids__/**THIS UID REMOVED**/dropbox/**THIS UID REMOVED**.dropbox/marriott%20marco%20island%20po#20-6402.pdf</p></body></html>
... I guessed that the "#" in the file referenced in the last paragraph was the problem. so I found the attached file in the calendar and deleted it. This solved the problem and returned the calendar to normal operation.
