@rfc2822 Thanks a lot for your quick reply! You were right: Having edited the affected entry in the nextcloud GUI, subsequent edits one the mobile phone synced without a problem.
Thanks again! Christoph
Since DAVdroid 1.2.2 the setup of task synchronization fails with OpenTasks installed on Android 5.1.
Steps to reproduce:
Thanks for the report. This has been fixed with eb70f8690f1b04a1104d3729843ddcf881fa8ee0 and should be working again in DAVdroid 1.2.3.