Synology NAS calendar sync fails for tasks

WildEnte

I've been using my Synology NAS as my primary calendar server for the longest time, syncing events and tasks. This has worked well until recently. Sadly I can't say when it stopped working well - definitely before the upgrade to 6.7.

I have two issues with the error message (see below):

the error message does not state which of my calendars is having a sync issue. I found out by spotting the task title and figuring out which calendar that task belonged to
there is no indication what I could do to fix it.

The issue seems to be one way: if I create a task on the synology web calendar, Vivaldi shows it in the tasks panel and I can mark those tasks as done, which syncs fine. But when I create a task in Vivaldi, I get the error and the Synology web calendar doesn't show the task. I have some older tasks which I definitely created in Vivaldi, which also show up in the error messages.
If I go to the Calendar status and hit "Accounts - refresh" I get a green checkmark that everything is a-ok.

Ideas what I could test to fix the issue, or to have enough info for a worthwhile bug report?

This is the error message (title and user name edited):

[caldav]Calendar. Error synching updates to server. Changes not saved. 412 {"alarmId":"0","allDay":false,"calendarId":"4","categories":"","complete":true,"completed":0,"component_class":"","deletePending":false,"description":"","end_recurring":0,"etag":"6ea24f008c03afa3380cfe8bb3560710","eventTypeId":"0","href":"/caldav.php/MyUserName/bbcfzaxr/7a4d3433-f83d-4ddb-865d-6cc2b7bb5bb0.ics","ical":"","id":"1356","invites":[],"is_recurring":false,"location":"","notifications":[{"delay":0,"description":"","event_id":"1356","id":"622","name":"","period":0,"when":1800000}],"organizer":"","percentage_complete":0,"priority":0,"recurrenceExceptions":[],"rrule":"","sequence":0,"status":"","syncPending":true,"task":true,"timezone":"Europe/Berlin","title":"Title of the task that doesn't sync","trash":false,"trash_time":0,"uid":"7a4d3433-f83d-4ddb-865d-6cc2b7bb5bb0","url":"","start":0,"end":0,"attachment":[]}

EDIT: a new profile seems to work immediately. Something wrong with my old profile? ... 6.7.3329.19

eggert

I assume this has to do with a regression from 6.6 which caused us not to save the url that the server returned for calendar items. We attempted to fix it by generating the url locally, but it might not work with this particular server.
It's just a theory though and I might be totally wrong.