• have a node named "toBeDeleted"
• set up a matrix job with this node and others
• delete the "toBeDeleted" node
• execute the matrix job -> it will still try to execute on the "toBeDeleted" node

to clean up the problem you need to set up a new node with the same name and first clean the matrix job and than delete the node again, because if a node is deleted the node cannot be found in the label tree in the matrix configuration, but the configuration still holds the old node.