The response from mailbox.org was:
mit diesen Infos können am Ehesten die Entwickler von DavDroid etwas anfangen. Wenn diese uns sagen können, welches Verhalten vom Open-Xchange Server erwartet wird, können wir das gerne an die Entwickler der OX App Suite weiterleiten.
Here is (hopefully the relevant part of) the error log:
SYNCHRONIZATION INFO
Synchronization phase: 4
Account name: ########@mailbox.org
Authority: org.dmfs.tasks
HTTP REQUEST:
PUT /caldav/44/C60E3142-8CFB-4C53-83EC-3FB0584A7DD5.ics
Content-Length: 377
Content-Type: text/calendar;charset=utf-8
If-Match: "http://www.open-xchange.com/etags/56-1461533166327"
BEGIN:VCALENDAR[CR][LF]
VERSION:2.0[CR][LF]
PRODID:+//IDN bitfire.at//DAVdroid/1.2-gplay ical4android ical4j/2.x[CR][LF]
BEGIN:VTODO[CR][LF]
DTSTAMP:20160715T160246Z[CR][LF]
UID:C60E3142-8CFB-4C53-83EC-3FB0584A7DD5[CR][LF]
SEQUENCE:1[CR][LF]
CREATED:20151227T195237Z[CR][LF]
LAST-MODIFIED:20160715T160145Z[CR][LF]
SUMMARY:Butter[CR][LF]
CLASS:PUBLIC[CR][LF]
STATUS:NEEDS-ACTION[CR][LF]
DTSTART;VALUE=DATE:20151228[CR][LF]
PERCENT-COMPLETE:0[CR][LF]
END:VTODO[CR][LF]
END:VCALENDAR[CR][LF]
HTTP RESPONSE:
http/1.1 500 Internal Server Error
Connection: close
Content-Length: 0
Content-Type: text/calendar
Date: 2016-07-15T16:02:48Z
Server: Openexchange WebDAV
Strict-Transport-Security: max-age=15768000
EXCEPTION:
at.bitfire.dav4android.exception.HttpException: 500 Internal Server Error
at at.bitfire.dav4android.DavResource.checkStatus(DavResource.java:316)
at at.bitfire.dav4android.DavResource.checkStatus(DavResource.java:289)
at at.bitfire.dav4android.DavResource.put(DavResource.java:200)
at at.bitfire.davdroid.syncadapter.SyncManager.uploadDirty(SyncManager.java:319)
at at.bitfire.davdroid.syncadapter.SyncManager.performSync(SyncManager.java:148)
at at.bitfire.davdroid.syncadapter.TasksSyncAdapterService$SyncAdapter.onPerformSync(TasksSyncAdapterService.java:78)
at android.content.AbstractThreadedSyncAdapter$SyncThread.run(AbstractThreadedSyncAdapter.java:259)
SOFTWARE INFORMATION
DAVdroid version: 1.2-gplay (109) Sat Jul 02 12:34:34 MESZ 2016
Installed from: com.android.vending
JB Workaround installed: no
CONFIGURATION
System-wide synchronization: automatically
Account: ########@mailbox.org
Address book sync. interval: 1440 min
Calendar sync. interval: 1440 min
OpenTasks sync. interval: 1440 min
Preemptive auth: true
WiFi only: false
[CardDAV] Contact group method: CATEGORIES
RFC 6868 encoding: true
[CalDAV] Time range (past days): 90
Manage calendar colors: true
SQLITE DUMP
android_metadata
| locale |
| de_DE |
----------
settings
| setting | value |
----------
services
| _id | accountName | service | principal |
| 1 | ########@mailbox.org | carddav | <null> |
| 2 | ########@mailbox.org | caldav | <null> |
----------
sqlite_sequence
| name | seq |
| services | 2 |
| collections | 52 |
| homesets | 9 |
----------
homesets
| _id | serviceID | url |
| 4 | 1 | https://dav.mailbox.org/carddav/ |
| 9 | 2 | https://dav.mailbox.org/caldav/ |
----------
collections
| _id | serviceID | url | readOnly | displayName | description | color |
timezone | supportsVEVENT | supportsVTODO | sync |
…
SYSTEM INFORMATION
Android version: 5.1.1
