Database in use was PostgreSQL. Users and groups are LDAP defined.
Although the admin console allows upload of icon/avatar at Server→Bookmarks→Goup Chat Bookmarks→Edit Bookmark→Icon/Avatar→Select File, the loaded content is displayed, but not saved by pressing the “Save Changes” button.
After leaving the edit page and resuming back the Icon/Avatar field is empty again.
Bookmarks seem to be something related but storage independent of users and groups, hence LDAP restrictions should not apply.