We have big problems with EM client and nextcloud calendar.
Claude’s info about the problem.
It’s an eM Client bug, and Nextcloud turns it into a mail storm. Person 1 loop is still running.
Person 1 loop, live now. Since 09:23 her eM Client 11.0.1201.0 has been re-uploading one event in her personal calendar about every 5 seconds: PUT, GET, PUT, GET. By 12:38 that was 1,341 uploads, about 13 a minute. With two attendees that’s about 2,700 mails so far, growing by about 26 a minute. The fastest stop is for Person 1 to quit eM Client or turn off that calendar account in it. From the server side you can revoke that client’s login token on the Nextcloud host. I haven’t changed anything.
Why it’s the client
Only eM Client 11.0.1201.0 writes the event, in a strict upload/fetch cycle. No other client and no Nextcloud background job touches it.
Person 2 case was the same. On 19 Oct, the day eM Client 11.0.1201.0 first appeared on his machine, it re-uploaded one event (6007B175-….ics) 111 times. It stopped on 20 Oct at 08:40, after his other installation (eM Client 10.4) saved that event once.
The server has run Nextcloud 34.0.3 since 19 Aug. eM Client 10.4 synced these calendars for weeks without looping. Person 1 got 11.0.1201.0 on 16 Oct; Person2 got it on 19 Oct and looped the same day.
In 14 days of access logs, no other client shows this pattern. Four users run 11.0.1201.0, so two more are at risk.
How the loop Works
eM Client uploads the event.
Nextcloud sends the invitations and records the delivery status on each attendee in the stored copy. That’s normal CalDAV scheduling behaviour. Because the stored copy now differs from what the client sent, the server replies without an ETag (a version tag), which tells the client to fetch the stored copy.
eM Client fetches it, but instead of keeping the server’s copy it uploads its own version again with a new last-modified time. Then the cycle repeats. Version 10.4 kept the server’s copy.
Nextcloud’s part: why a sync glitch becomes a mail storm
Nextcloud 34 replaced the stock scheduling logic with its own, which marks every update as worth an invitation. It never checks whether date, time, location or title changed. The only remaining check passes whenever the last-modified time or the event’s revision number changes. So a re-save that changes nothing else re-invites every attendee. The stock logic would have sent nothing.
Nextcloud sends invitations through the organizer’s own account in the Mail app, which is on by default. Both users have Mail-app accounts, so the invites go out under their own SMTP login. That’s why it looks like Person 1 sent 2,000+ mails. The Mail app also puts a copy of each one in her Sent folder.
This deserves an upstream report, but it isn’t the cause. Patching Nextcloud’s core files would break its integrity check and get overwritten on upgrade.
Can you check this and fix the problem?