When you add an object relation (view "content/browse"), the node you want to add is blocked (the checkbox is disabled).

Why?
The NodeID of the node to be added is contained in the NodeID of the node being edited.

The error is in ezpublish_legacy/kernel/content/relation_edit.php at the 72 row:

$assignedNodesIDs = $node->attribute( 'node_id' );

My patch, add the square brackets:

$assignedNodesIDs[] = $node->attribute( 'node_id' );