Calendar Sync problem with OpenSync App

Hey guys,
I’m not sure if this is the right forum for this problem, but I didn’t find a better one so I will try it here.
Cause of some Sync Problems after an Android Update with another CalDAV App I changed to the OpenSync App about 4 month ago. For a long time everything worked well but since about half a week I get error notifications which are declaring, that the Calendar couldn’t be synchronized. Contacts Sync with App and Kalendar Sync with CalDAV at KOrganizer are working well.
To try to fix the problem I deactivated the android performance optimization for the app (hint was displayed when opening the app) and configured the same account another time. Both haven’t worked.
Cause the app also gives a debug information which I don’t understand I want to ask you if you have an idea what the problem is. The debug info can be found under this entry.

Greetings from Northern Germany,
Maximilian

=== debug.txt ===
-> cause I’m not allowed to post more than two links in this forum I substituted all links with ###link###

--- BEGIN DEBUG INFO ---
SYNCHRONIZATION INFO
Synchronization phase: 4
Account name: maximilian
Authority: com.android.calendar

EXCEPTION:
java.lang.NullPointerException: Attempt to invoke virtual method 'void java.util.Calendar.setTimeZone(java.util.TimeZone)' on a null object reference
	at java.text.DateFormat.setTimeZone(DateFormat.java:690)
	at net.fortuna.ical4j.model.Iso8601.<init>(Iso8601.java:77)
	at net.fortuna.ical4j.model.Date.<init>(Date.java:150)
	at net.fortuna.ical4j.model.DateTime.<init>(DateTime.java:252)
	at at.bitfire.ical4android.AndroidEvent.populateEvent(AndroidEvent.java:185)
	at com.deependhulla.opensync.resource.LocalEvent.populateEvent(LocalEvent.java:70)
	at at.bitfire.ical4android.AndroidEvent.getEvent(AndroidEvent.java:131)
	at com.deependhulla.opensync.resource.LocalCalendar.getDirty(LocalCalendar.java:135)
	at com.deependhulla.opensync.syncadapter.SyncManager.uploadDirty(SyncManager.java:326)
	at com.deependhulla.opensync.syncadapter.SyncManager.performSync(SyncManager.java:156)
	at com.deependhulla.opensync.syncadapter.CalendarsSyncAdapterService$SyncAdapter.onPerformSync(CalendarsSyncAdapterService.java:70)
	at android.content.AbstractThreadedSyncAdapter$SyncThread.run(AbstractThreadedSyncAdapter.java:272)

SOFTWARE INFORMATION
OpenSync version: 1.5.0.3-ose (145) Tue Apr 18 09:03:24 GMT+02:00 2017
Installed from: com.android.vending
JB Workaround installed: no

CONFIGURATION
Power saving disabled: no
android.permission.READ_CONTACTS permission: granted
android.permission.WRITE_CONTACTS permission: granted
android.permission.READ_CALENDAR permission: granted
android.permission.WRITE_CALENDAR permission: granted
org.dmfs.permission.READ_TASKS permission: denied
org.dmfs.permission.WRITE_TASKS permission: denied
System-wide synchronization: automatically
Account: maximilian
  Address book sync. interval: 240 min
  Calendar     sync. interval: 240 min
  OpenTasks    sync. interval: —
  WiFi only: false
  [CardDAV] Contact group method: GROUP_VCARDS
  [CalDAV] Time range (past days): 90
           Manage calendar colors: true
Address book account: Contacts (maximilian JQ)
  Main account: Account {name=maximilian, type=deependhulla.com.davdroid}
  URL: ###link###
  Sync automatically: true

SQLITE DUMP
android_metadata
	|  locale |
	|  de_DE |
----------
settings
	|  setting | value |
----------
services
	|  _id | accountName | service | principal |
	|  1 | maximilian | carddav | ###link### |
	|  2 | maximilian | caldav | ###link### |
----------
sqlite_sequence
	|  name | seq |
	|  services | 2 |
	|  homesets | 2 |
	|  collections | 4 |
----------
homesets
	|  _id | serviceID | url |
	|  1 | 2 | ###link### |
	|  2 | 1 | ###link### |
----------
collections
	|  _id | serviceID | url | readOnly | displayName | description | color | timezone | supportsVEVENT | supportsVTODO | sync |
	|  1 | 2 | ###link### | <null> | -8853644 | <null> | 1 | 1 | 1 |
	|  2 | 2 | ###link### | 1 | Contact birthdays | <null> | -54 | <null> | 1 | 1 | 1 |
	|  3 | 2 | ###link### | 0 | Personal | <null> | <null> | <null> | 1 | 1 | 1 |
	|  4 | 1 | ###link### | 0 | Contacts | <null> | <null> | <null> | <null> | <null> | 1 |
----------

SYSTEM INFORMATION
Android version: 8.0.0 (R16NW.G960FXXU2BRGA)
Device: Samsung SM-G960F (starlte)

--- END DEBUG INFO ---

Looks more like an issue related to davdroid - please ask for help in https://forums.bitfire.at/category/4/davdroid

If it turns out to be a server issue just ping me and I’ll help out!

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