If you encounter a synchronization error when saving or sending a calendar task from eM Client to Nextcloud, the issue might be due to an incorrect time zone.
You can test this by creating a calendar task with the “all-day” parameter. If this works, you can resolve the issue on the Nextcloud side by applying a procedure that fixes the time zone for Nextcloud. Alternatively, you can either use all-day tasks or set the time to UTC (Coordinated Universal Time).
It is just my advice, not a question :-). But It took us a long time to solve it.