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Sorry to hear you’re facing problems 
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Nextcloud version (eg, 20.0.5): 27.1.6
Operating system and version (eg, Ubuntu 20.04): CentOS 7
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.6
PHP version (eg, 7.4): 8.2.16
The issue you are facing:
Having strange issues with synchronizing calendars with/to NextCloud from
- Thunderbird on Windows
- Calendar on Android via DAVx5
In the clients, some events are disappearing or not getting synced, without any apparent pattern. Other/most events are fine.
Have reviewed NextCloud Admin->Logging, nextcloud.log, Apache logs, system logs, and no relevant diagnostics were found.
Just looking for some advice/suggestions how to go deeper into debugging the issue.
Is there a way to enable debug logs for the CalDAV sync process on the Nexcloud side without enabling debug logging for everything?
Is there any kind of caching in CalDAV that can be cleared?