with 4.3 the List Files node has been replaced and now creates a new type of cell, i.e., a path cell. The table row to variable loop start node creates a path flow variable from this cell which in turn cannot be used by nodes that haven’t been ported to the new file handling framework (which this cell and variable type are part of).
There are two ways to approach this problem. Either you use the CSV Reader, which nearly has all the options the File Reader has (and a couple more) while being way faster or you use the Path To String (Variable) node to convert the variable to the type you need in order to configure the File Reader.
Hope this solves your issue and sorry for the inconveniences. It will take us some more time till we ported all reader and writer nodes to the new framework.