Nextcloud CalDav sync problem - Invalid Format

Hi everybody,
I tried to find similar topics here, but unfortunately to no avail. Many people seem to successfully use Nextcloud calendar with emClient. (Which is great and I’m planning to do that as well :wink:)

Here’s my setup:
Nextcloud 22.2.0 on managed server
Created a user, added the calendar app. I’m using the empty “personal” calendar.
Got the private link from nextcloud, in emClient went “Tools” - “Subscribe to Internet Calendar”, pasted URL and filled in Username and Password.

Then I get the following error:
[Web Calendar]Synchronizing folder 'Internet Calendars/' failed due to the following error: One of the identified items was in an invalid format.

Full error log:
15:57:02 Internet Calendars Web Calendar Synchronizing folder '/'
15:57:05 Internet Calendars [Web Calendar] MailClient.Storage.Application.OperationException: Synchronizing folder 'Internet Calendars/' failed due to the following error: One of the identified items was in an invalid format.

  ---> System.FormatException: One of the identified items was in an invalid format.
                at MailClient.VObject.CalendarReader.LoadCalendar(Calendar calendar)
                at MailClient.VObject.Calendar..ctor(TextReader reader)
                at MailClient.Protocols.WebCal.WebCalCalendarSynchronizer.Synchronize(IItemSynchronizeContext1 synchronizeContext, Folder folder, CancellationToken cancellationToken)
            at MailClient.Protocols.Common.ItemSynchronizer2.<>c__DisplayClass37_1.<EnqueueSynchronize>b__6(WorkerStatus status, CancellationToken ct)
            at MailClient.Protocols.WebCal.WebCalGenericCommand.Execute(WorkerStatus status)
            --- End of inner exception stack trace ---
           GUIStatus_Exception_Reported = True

Syncing the calendar with Thunderbird works just fine. Strange is that the calendar is currently empty, so I’m not quite sure which item could have an invalid format. Maybe someone can help?

All the best

Edit: I’m using eMClient version 8.2.1659

Hi Olav,

One of our customers has the same problem that you did. (We’ve been using Nextcloud with eM Client for a couple of years with no issues.)

Did you ever happen to find a solution?


Hello in order to fix it we would need to reproduce it, so we would need a calendar where this is happening or at least full CalDAV logs sent to [email protected]. I believe we’ll be able to fix it or workaround it pretty quickly once we have it reproduced.

Hello Michal, thanks very much for your speedy reply. As it turns out, there is no problem with eM Client. When I contacted the customer to generate logs for you, I connected to his machine remotely, and discovered that he had been trying to configure the calendar incorrectly.

Rather than going through Accounts → Add Account → Calendar → CalDAV, he had made the mistake of trying Tools → “Subscribe to Internet Calendar”. I explained the difference between CalDAV and .ics to him. The problem is now resolved.

Thanks again for your availability.