Amazing, thanks a lot Simon!
Two solutions:
- Put Path to String (Variable) node in between the second (after creating the Flow Variable) and third (File Reader) node, or;
- Put Path to String node in between the first (List Files/Folders) and second (Table Row to Variable) node.
Again, thank you for pointing me to this node. 
Edit: I needed to add the prefix “file:\” in front of Path in order to be able to get results from the File Reader node. I encountered the same error as in Problem with the File Reader Node when using workflow variables (again) - #2 by MH.
I ended up using:
- List Files/Folders
- Constant Value Column (for “file:\”)
- String Manipulation (for joining the two columns)
- Table Row to Variable
- Path to String (Variable)
- File Reader