When creating a workflow eighter for local client or a server the documents to provide might change from case to case. If not filled with valid files, the two mentioned nodes fail. Especially on the server this is anoying, for leaving the form (forward or backward) will cause the whole process to fail.
For the path must be valid even the selection of an output file to create (so not existing yet) fails and makes the node unusable… 
When changing the nodes, ensure the option “required” is available in the node’s configuration menue (as checkbox) as well as already existing on the variable selection page.
Thanks!