I started having this issue in SW 2017 SP4.1. I'm still having this issue after recently updating to SW 2017 SP5.0.
In any drawing, after I go into Sheet Properties and select a sheet format other than the one loaded with the template, then try to save the file, I get the error: "An unknown error occurred while accessing <file path>."
Only option is to click 'OK' and goes back to the drawing un-saved. I've tried saving to different directories, it creates the file but still throws the error. Opening the newly-created file gives me "SOLIDWORKS encountered a serious problem in the file <file path>. Please contact your local SOLIDWORKS support representative for help in troubleshooting this problem."
This is a very debilitating issue and any help would be greatly appreciated.