[Bug]: Webdav availability request gives malformed response

[GVodyanov]

⚠️ 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 OR Nextcloud Community Forum (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

Webdav availability request gives malformed response, especially when a user doesn't have any availability slots configured so far

Steps to reproduce

1. Create a new user
2. Go to availability settings
3. See "Failed to load availability" (first issue)
4. Set some availability slots and save
5. Reload the page
6. The availability slots aren't shown

Expected behavior

No errors and slots are saved

Installation method

Other Community project

Nextcloud Server version

master

Operating system

Other

PHP engine version

None

Web server

None

Database engine version

None

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

Fresh Nextcloud Server install

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

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

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response