[Bug]: CalDAV issues collection ticket

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Collecting all bugs that @st3iny and I found when debugging the scheduling:

1. When deleting events from the trashbin a cancelled email is being sent from scheduling
2. When manually cancelling a recurring event, the sent email will be a method="REQUEST" email instead of a method="CANCEL".
3. Something's funky with processing calendar objects from an external client. Could possibly be the ACLs. "Submit my changes anyway" pops up in Thunderbird.
4. Cannot accept recurrences from external client (internal we have not supported that ... yet)

Steps to reproduce

1. Install Thunderbird
2. Add Calendar in Thunderbird
3. Play around with accepting and declining events generated in Nextcloud

Expected behavior

THe correct email method is set
Events from trashbin do not generate emails
etc

Also, stumbled upon this bug when testing too: https://bugzilla.mozilla.org/show_bug.cgi?id=1406402

Installation method

No response

Operating system

No response

PHP engine version

No response

Web server

No response

Database engine version

No response

Is this bug present after an update or on a fresh install?

No response

Are you using the Nextcloud Server Encryption module?

No response

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

see c.nc.com config for that

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start by reproducing the reported behaviors with Thunderbird and Nextcloud Calendar, covering trashbin deletion, manual cancellation, external-client changes, and recurring events. The issue does not name source files or tests, and completion criteria are only described broadly as correcting email methods and preventing unwanted emails.