Looks like a bug in the dav app. A quick glance suggests that the beforeDelete() method in server/ChunkingV2Plugin.php at master · nextcloud/server · GitHub is missing a call to $this->checkPrerequisites(); (which the other three “before” and “after” methods has in them). When getUploadStorage() is called in that function, there’s a missing value and the error occurs.
I think you should open an issue at Issues · nextcloud/server · GitHub . I didn’t find an existing one.