Your file-path parameter has an incorrect structure. You need to use the so-called "platform independent path".
The best way to find out how it needs to look is to open a file from that folder and then run this code from the console:
It will output the file-path in the correct structure that you need to use in your script.
Take a look at the documentation for Doc.extractPages(): Acrobat DC SDK Documentation
It shows in the examples how the path needs to be specified as a device independent path (in your case, it would be "/server/folder/subfolder/filename.pdf"). You can find out more about device independent paths in the PDF specification.
There is one other useful bit of information in the documentation: When you do not specify the output path, the method will actually return the document object for the newly created document: "If cPath is not specified, returns the Doc for the new document; otherwise, returns the null object."
With that document object, you can then further modify the document and save it via the Doc.saveAs() method (but again, the path needs to be specified as device independent path).
That took care of it. Thank you!