If it has worked in the past and you have made no changes, the problem would not appear to be with how the calculation is written. Either the values in the referenced fields are producing an invalid file name or file path or there is a permissions or available space issue with your computer.
First thing that I'd check is the value in EquipmentDescription at the time the step executed. Could there be a character in that field that isn't allowed as part of a file name?
"File:"&Get(DesktopPath) (Sales::EquipmentDescription&" "&"Document.pdf")
Looks like something that should produce a syntax error when you try to close the dialog box. Is there a missing & in what you typed?
As soon as you related about the possibility of a syntax error it alerted me to the pdf file itself and sure enough
the document.pdf was saving a "/" because a user was trying to split a word with /.
Thanks as always.
Filter can be a very helpful function for removing illegal characters from the file name part of your calculation.