Collabora / Nextcloud: Cannot save as guest user

When sharing a (Text / Calc-) document via Nextcloud with a public link, the (guest) user cannot save the document in Collabora:
image

The document was shared like this:
image

However, with the user that owns the file can save it in Collabora online without any problems. It does not matter if the file is in a folder on external storage (AWS S3) or directly in Nextcloud’s root (which is local storage).
It does not matter if the file was created externally and synced via the Nextcloud client or created directly from Nextcloud web interface.

System:
Collabora 4.2.0.1
Nextcloud 16.0.5-0
(both installed via AppCenter as Docker images)
Kopano (old install without docker)

URLs:
https://kopano.example.com (redirection to /webapp)
https://nextcloud.example.com/nextcloud
Nextcloud --> Settings --> Collabora --> URL of Collabora server: https://nextcloud.example.com
Both URLs have a Let’s Encrypt cert.

Any ideas what the cause could be?