[BUG] Daily calendar backup leads to cancelled/resent event invites for a Microsoft365 account

[tofublock]

⚠️ Before posting ⚠️

• This is a bug, not a question or an enhancement.
• I've searched for similar issues and didn't find a duplicate.
• I've written a clear and descriptive title for this issue, not just "Bug" or "Crash".
• I agree to follow Nextcloud's Code of Conduct.

Steps to reproduce

1. Sync a Microsoft365 calendar into the Android calendar provider (have tested this with the official Outlook app and Nine Mail)
2. Enable daily calendar backup
3. Create a calendar event in the Microsoft365 calendar and set attendees so the event is sent out to them
4. After daily backup, check sent mails on Microsoft365 account- events cancellations get sent to attendees and sometimes rescheduled afterwards. Sometimes cancellations are being sent with no rescheduling.

Expected behaviour

Daily calendar backups should quietly take a backup and not interfere with the calendar in any way otherwise (leaast of all lead to event cancellations.)

Actual behaviour

For a synced Microsoft365 calendar, the daily backup leads to cancellations being sent to recipients of calendar invites. I have not tested if this is true for events in the Nextcloud calendar as they don't get sent out on my instance.

Android version

13

Device brand and model

Google Pixel 6a

Stock or custom OS?

Custom (explain in "additional information")

Nextcloud android app version

3.25.0

Nextcloud server version

26.0.3

Using a reverse proxy?

Yes

Android logs

No response

Server error logs

No response

Additional information

Phone is on GrapheneOS, but this issue has plagued me on my previous phone with various Android versions and ROMs. I have changed mail apps, accused the MS Teams app, called MS support multiple times...
This morning, I finally caught the brief backup notification while it was ongoing and sure enough- a test event I had created got resent at exactly the same time right after.
Very happy I found the culprit, but having client meetings automatically cancelled is actually horrible and has led to uncomfortable situations...

[tofublock]

To add that this bug started to occur around half a year ago if I had to guess.

[tofublock]

I was able to trace this back further: Backing up the calendar using the Calendar Import Export app the Nextcloud app borrows the calendar manipulation from, I see the same behaviour.

The code seems to do something with the calendar entries that makes it look like they have been changed so updated invites are being sent.
Development is not very active in the sufficientlysecure repo, but I'll try to raise an issue there.

[joshtrichards]

I'm wondering if this is a CalDav matter in Server rather than the Android client.

For example, nextcloud/server#34778

[tofublock]

Hm, possible. I didn't question it after I found the "make problems stop" switch in the Android app. But you're right, might well be server side.