So what i did that solved the problem, is that I downloaded the latest Arduino IDE, titled "Windows ZIP file for non admin install". Then I unzipped the download, and I copied all of the files of the new IDE and pasted them into the old installation. But I did not replace anything. i.e. For each file that existed, I kept the old file.

~400 new files were copied. Now it compiles without error.

BTW Since this has been a blind workaround, I will not accept this as the final answer.

So what I did that solved the problem was, I downloaded the latest Arduino IDE, titled "Windows ZIP file for non admin install". Then I unzipped the download, and I copied all of the files of the new IDE and pasted them into the old installation. But I did not replace anything. i.e. For each file that existed, I kept the old file.

~400 new files were copied. Now it compiles without error.

BTW since this has been a blind workaround, I will not accept this as the final answer.