I posted the problem to EGroupware as well, but did not get an answer there.
I finally chose to swith to another CalDAV-Sync app which seems to work better with EGroupware.
I am having trouble synching my Nextcloud calendar and contacts. I attempted to read up on this, yet got nowhere. Earlier I opened a ticket on the Nextcloud support forum (https://help.nextcloud.com/t/android-outlook-sync-error-sabre-issue/51633), yet they were unable to help suggested I open a ticket on this forum.
While not wanting to duplicate, please find below a copy of what I posted on the Nextcloud forum. Thanks in advance!
"Nextcloud version (eg, 12.0.2): Nextcloud 15.0.5
Operating system and version (eg, Ubuntu 17.04): Win 10 Pro
Apache or nginx version (eg, Apache 2.4.25): ?
PHP version (eg, 7.1): 7.3
The issue you are facing:
It is not possible to sync the calendar and contacts using both davx5 and the Outlook plug-in. However, it was possible to install the Nextcloud app on Android and for the app to connect to the server.
Is this the first time you’ve seen this error? (Y/N):
Steps to replicate it:
Install Nextcloud server. Set up two-factor authentication and generate app specific passwords. Install davx5 and Outlook plug-in.
The output of your Nextcloud log in Admin > Logging:
[webdav] Fatal: Sabre\DAV\Exception\ReportNotSupported: at <<closure>>
from IP address by My Nextcloud admin username at 2019-04-14T17:15:29+00:00
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
Am happy to share this later since I cannot determine which information might compromise the system if posted on the internet.
The output of your Apache/nginx/system log in /var/log/____:
Unsure where I can find the Apache log. However, while looking for the log file, I noticed that there is still an old owncloud installation on the server, which is seemingly still generating log files. I cannot see that installation in the app installer, so will ask the administrator to manually remove it. "
Although there is no debug info, it seems that Nextcloud returns an error when DAVx5 requests the events/contacts. Is there any indication that this could be a DAVx5 problem?