Welcome to the mailbox user forum

New Topic

mailbox user forum Topic Problem Calendar

Mailbox calDAV implementation not RFC compliant.

Christopher Online shared this problem ago

Published

Trying to sync a repeating task to the mailbox.org caldav server from an iOS client app and run into the error which you can see at the very end of this message. Furthermore I received the feedback that the mailbox.org caldav server is apparently it RFC compliant:

“ I can see the CalDAV server in particular is picky about BusyCal sending DTSTART == DUE (this isn't strictly RFC compliant but many servers out there would fail to work if they weren't equal). We'll have to add a workaround for the next upcoming update for this to work.”

I am really disappointed as a paying customer by mailbox.org support. They are unwilling to help me and let me think that I am in the wrong for using a third party client. Using the ugly mailbox.org web interface is simply not an option especially on the mobile phone and since it is the mailbox.org sever’s fault not sticking to the RFC standard. And waiting for such a reply for almost three weeks is ridiculous.

Error log:

026-04-26 12:37:25 B -1- CalDAVSyncTask (31187546-3C9E-4F57-9EAF-42D3795C9422): push UPDATE event Asdf daily (DAE7DFA7-8561-4B6B-A239-4FDB6E293A8D) to calendar Repeating (etag: ignore)

2026-04-26 12:37:25 M -2- send request =>

PUT https://dav.mailbox.org/caldav/NTE/DAE7DFA7-8561-4B6B-A239-4FDB6E293A8D.ics

User-Agent: BusyCal-6.6.0 (Calendar) Mac OS X/13.0.1

Content-Type: text/calendar

Authorization: <>

Cache-Control: no-cache

BEGIN:VCALENDAR

VERSION:2.0

PRODID:-//BusyApps//BusyCaliOS 6.6.0//EN

CALSCALE:GREGORIAN

BEGIN:VTODO

DTSTART;VALUE=DATE:20260426

DUE;VALUE=DATE:20260426

RRULE:FREQ=WEEKLY;INTERVAL=2

SUMMARY:Asdf daily

UID:DAE7DFA7-8561-4B6B-A239-4FDB6E293A8D

SEQUENCE:1

DTSTAMP:20260426T103725Z

CREATED:20260422T121912Z

LAST-MODIFIED:20260426T103722Z

X-BUSYMAC-LASTMODBY:iPhone

END:VTODO

END:VCALENDAR

2026-04-26 12:37:25 B -1- received 400 response =>

HTTP 400

URL: https://dav.mailbox.org/caldav/NTE/DAE7DFA7-8561-4B6B-A239-4FDB6E293A8D.ics

Content-Type: text/xml; charset=UTF-8

x-powered-by: Open-Xchange

x-xss-protection: 1; mode=block

x-download-options: noopen

Content-Encoding: gzip

x-permitted-cross-domain-policies: none

referrer-policy: no-referrer

Date: Sun, 26 Apr 2026 10:37:25 GMT

x-robots-tag: noindex, nofollow

Strict-Transport-Security: max-age=15552000; includeSubDomains; preload;

feature-policy: accelerometer 'none'; camera 'none'; gyroscope 'none'; magnetometer 'none'; microphone 'none'; usb 'none';

x-content-type-options: nosniff

Vary: Accept-Encoding

<?xml version="1.0" encoding="UTF-8"?>

<D:error xmlns:D="DAV:" xmlns:OX="http://ox.io/ns/">

<OX:error>An error occurred inside the server which prevented it from fulfilling the request.</OX:error>

<OX:category>ERROR</OX:category>

<OX:code>WEBDAV-1000</OX:code>

<OX:error_id>2030185343-1865961</OX:error_id>

<OX:error_desc>A WebDAV error occurred. (HTTP 400)</OX:error_desc>

</D:error>

2026-04-26 12:37:25 B -0- CalDAVSyncTask (31187546-3C9E-4F57-9EAF-42D3795C9422): server returned 400, retrying with compatibility export for event Asdf daily (DAE7DFA7-8561-4B6B-A239-4FDB6E293A8D) [omit equal task due]

2026-04-26 12:37:25 B -2- CalDAVSyncTask (31187546-3C9E-4F57-9EAF-42D3795C9422): will try and update again to: (null)

2026-04-26 12:37:25 B -2- CalDAVSyncTask (31187546-3C9E-4F57-9EAF-42D3795C9422): updated SEQUENCE for event Asdf daily (DAE7DFA7-8561-4B6B-A239-4FDB6E293A8D) to: 1

2026-04-26 12:37:25 B -1- CalDAVSyncTask (31187546-3C9E-4F57-9EAF-42D3795C9422): retrying push without equal task DUE for event Asdf daily (DAE7DFA7-8561-4B6B-A239-4FDB6E293A8D)

2026-04-26 12:37:25 B -1- CalDAVSyncTask (31187546-3C9E-4F57-9EAF-42D3795C9422): push UPDATE event Asdf daily (DAE7DFA7-8561-4B6B-A239-4FDB6E293A8D) to calendar Repeating (etag: ignore)