With 5.6 the collections API has been entirely re-written. This brings some conceptual changes and a larger feature set to collections.
The main difference between the old and new implementation is that a user is no longer created for external shares. Instead, the share impersonates the owner of the collections when the assets are accessed via a sharing token (link or e-mail).
On top of that, it adds the following features
More sharing options
Link (External)
Groups (Internal)
The expiration date for all shares
Revoking a share
Overview of who it is shared with
It is no longer possible to remove shared collections from your collections page.
For more information and a migration path, see the following documentation.
Do note that old links to collections no longer work after the update, even if the collection themselves has been migrated.
The collection migration script: