UMLR — Impossiblity to specify links for connected roles.

The specification says

[...]Associations, [...] specify links between any suitably-typed instance of the associated Classifiers, Connectors specify links between instances playing the connected roles [...]

A link specified by an Association can be modeled by an InstanceSpecification. A link only specified by a Connector cannot be modeled, since Connector is not a Classifier.

That means that no object-diagram can show the links between connected roles, when the Connector is not typed by an Association. It doesn't help, that the InstanceSpecification could be left without classifier. In this case, it would not be possible to define the linked instances, since an InstanceSpecification without classifier can't have Slots.