Second Contextual Filter on Calendar Module
Entity reference field can not provide content id to your view through contextual filter by default. You'll need to define a relationship to your entity reference field so that view is able to pick up Client node id ( content:nid) as contextual filter.

Remove contextual filter that you have now and add relationship to the entity reference field 'Client'. Then add contextual filter to Content:nid. In relationship setting select the alias of relationship that you added previously. In the section " When filter value is not available" check 'Provide default value -> raw value from url -> path component = 2'. Also check "use path alias".