I am using ModelBuilder within ArcMap 10.4 with the X-Tools Pro addon.
One of the last tools that is executed in my model is the "Export to KML" tool from the X-Tools Pro addon.
After a couple of iterations, it will export to KML on some input parameters (e.g. "3WN20") but I am finding some input parameters (e.g "1336PH1989") will not execute and display an 'Invalid File Name' error message. Whilst using "3WN20" all folders, file paths, and resulting shapefiles and KMZ's are created no worries.
I will add that I can successfully export "Layer to KML" using the inline variable substitution. However, the "Export to KML" tool allows me to categorise field types in folders in the exported KML. Hence, why I am using it.
Example of the error message is below: