Hi there,
keep in mind that windows 7 is a system that is now over 12 years old. In earlier versions older versions than windows 10 weren't even supported because of constant bugs. Since 0.5 these have stabilized, but they can still occur.
Does this loading error only occur on one save or on all save files? have you tried restarting with a new game and saving on day 15 to see if it was only a one time issue?
playing from external hard drive makes no difference btw, the save files are located in your users folder